Comfrey is an amazing perennial. To even think about potting this plant as a decoration is sad. One of the very few true 'companion plants' that actually work to pot them is selfish? Grins!
In zone 4a as a minimum, these plants in buckets will have to be brought indoors or their roots will be subjected to freezing temperatures in winter. Buckets of comfrey to line a driveway will look like buckets of weeds lining the driveway albeit pretty and kinda cool as you will have uniformity with just one plant. To use tar buckets is contraindicated IF you plan on using comfrey as a medicinal or for making mulch to use in the soils of edibles!
One of the coolest things about Comfrey is that it is an 'dynamic accumulator'. I just found that term...like Kale and brassicas, it absorbs nitrogen and heavy metals and stores these chemicals in its leaves. Clovers can grab and fix nitrogen from the air and store it in the clover's stems, leaves, roots. Comfrey doesn't fix nitrogen but it accumulates it as well as Pottassium. Pretty flowers, leaves, great medicinal properties (toxic for ingestion) but comfrey is best used as a decomposed mulch.
Comfrey needs to be planted in the soil of the garden. I would grow it as a crop and make decomposed mulch to dress all my plant beds. I'd be careful with vegetables as it also accumulates heavy metals from the atmosphere. Get a test of some of your leaves or decompose the leaves, mix them into the soil and then just get a soil test and highlight 'heavy metals' to be tested as well. Having lots of problems with heavy metals in our soils and water these days if you've been watching the news...
If you want something along your driveway, I would plant a hardy evergreen...no actually I would plant Salix purpurea 'nana' in a staggered row. I'd then plant the comfrey as little groups or sweeps in front and in back of the willow hedge. I'd add daylillies, a few spectacular orange or red or purple flowering specimens and group or sweep Stella de Oro daylilly, an evergreen daylilly that makes a tall dense ground cover to shade and cool the soil. It will also combine aesthetically with the specimen daylillies. The willows are bluish green with a dense growth of long, thin, copper stems that are gorgeous in the winter. The well pruned soft hedge actually glows in the winter sun!
Pruning to shape (think of marshmallows floating in hot chocolate but orderly) is actually fun to do and only 2 maybe 3 prunings/shearings per year to keep it a 2 1/2 foot to 3 foot tall hedge along your driveway. Like cutting hair easy. This shrub 'moves' in the wind akin to the grasses. I do these shrubs by hand shearing because it is so much fun to do. Start by grabbing all the stems in one hand, pull up perpendicular to the base of the shrub and cut at the height you want, let go and you've got your template. This willow will grow to 30' by 30' if you don't prune it. Pruning keeps these shrubs soft and dense. Even the 'nana' is meaningless! Pruning is so very much fun and a soft hedge of these is to die for. My opinion of course! I dug up and transported my blue arctic willow hedge 3 times to transplant at a new home. Beautiful. I should start posting some pictures.
Comfrey likes moist soils. Use the soil you've got along the driveway, double dig (make 6"x6" trenches between the driveway and the back side of your driveway plant beds) or you'll get soil and mulch all over your drive when it pours or when you water deeply. Start imbuing your beds alongside the driveway with lots of decomposed mulch. Set your bed to be at least 4' or even 5' wide keeping the plants well away from the driveway. Asphalt and concrete can really heat the soil nearby and most plants do not like too hot, too dry of a soil (that will have a high pH if your driveway is concrete). Asphalt is obviously a petroleum product and I wouldn't plant anything you intend to use medicinally or for food or eat anything edible (daylilly flowers for instance).
Are you using Symphytum officinale X asperum = uplandicum? Or Russian comfrey cultivar? That sterile cultivar Blocking 14? At least that cultivar isn't invasive but tougher to propagate. Whatever you are using for comfrey, the first year cut off those flowers and their stems! This helps them produce great roots and ensures they will come back the next year. If they are planted in the garden soil, NOT POTS.
As for using just mulch for potting soil, no. Mulch is not a substitute for soil (for pots I would use potting soil with mycorrhizae) just need to top the plant beds or potting soil with well decomposed organic matter. Comfrey is one of the best decomposed organics to use! Potting soil needs SOIL. The commercially bagged soils actually have very little soil added but there is soil nonetheless! Sure stuff will grow in pure mulch but if you know what you are doing you will use potting soil for pots and mulch the top once per year and keep the garden soil in the garden. Have you been following 4-K and Bamboo about potting soil?
Oh, one other caveat, comfrey loves moist soils which causes high humidity and rust and powdery mildew will be a huge problem. Planting alongside your driveway is good because there will be more air and the change in temperatures (because of a large surface area of hardscape) will cause quite a bit of airflow, which you will need. Don't plant too closely or at least thin your comfrey periodically. Always wear gloves when handling your comfrey.