When did you find linden seedlings... Spring? Fall? Are you talking about
new seedlings, or some that may have sprouted a year or two ago?

I'm curious if the
seeds need a wintering cycle thus sprouting in Spring, or not, thus sprouting
in Fall.

I like the rooting idea - for now, Fall here in Mass, I'd like to see what I can do
with the seeds.

For my area, basswood leaves are huge and more crinkled than linden.
Linden trees often have hundreds of seedlings underneath them. I've
never looked under basswood. I've pulled up a few linden sprouts and
transplanted them. They seem to be doing well, keeping my fingers
crossed that they'll survive winter.

If there's one you like in particular, consider taking a cutting in
spring and root it with some rooting hormones.