Plantpropagation.com is a commercially sponsored site, but they have this information for Japanese Maple:
Most Japanese Maple seeds ripen in the fall. Watch the tree and wait
for the seeds to turn brown. The seeds are ready to be harvested when
they are brown and can be easily removed from the tree. The seeds are
attached to a wing, it's best to break the wing off before storing or
planting the seeds.
Japanese Maple seeds have a very hard outer coating as do many
ornamental plants. Under natural conditions the seeds would have to be
on the ground for almost two years before they would germinate. All
that happens the first winter is the moisture softens the hard outer
shell, and the second winter germination is beginning to take place.
In order for all of this to happen in the proper sequence so the
seedlings actually sprout at a time of the year when freezing
temperatures or hot summer sun doesn't kill them, takes a tremendous
amount of luck. You can improve the odds by controlling some of these
conditions, and shorten the cycle.
Once you have picked the seeds and removed the wing just place them in
a paper bag and store them in a cool dry place until you are ready for
them. You don't want to plant your seeds out in the spring until the
danger of frost has past, here in the north May 15th is a safe bet. If
May 15th is your target date you should count backwards on the
calendar 100 days. That will take you to about February 5th if my math
On or about the 100th day prior to your target planting date, take the
seeds and place them in a Styrofoam cup or other container that will
withstand some hot water. Draw warm to hot water from your kitchen
faucet and pour it over the seeds. Most of the seeds will float, just
leave them in the water overnight as the water cools down. 24 hours
later most of the seeds will have settled to the bottom of the cup,
drain off the water. Place the seeds in a plastic bag with a mixture
of sand and peat or other suitable mix. Even light potting soil will
work. The peat or soil should be moist, but not soaking wet. Poke some
holes in the bag so there is some air circulation, and place the bag
in your refrigerator for a period of 100 days.