Importing preferences from a Lion time machine backup to Snow Leopard
This isn't something that Apple tests well (or perhaps at all) - so you might have some crashes, but in general these are minor or confined to one app.

If you do the downgrade, just be systematic about watching for crashes, keep all the backups. You can always trash the preferences if you have odd feelings or just trash them all after the restore to start "clean" with Snow Leopard defaults but all of your Lion data.

It would be much better to export the data from Lion and restore from your last Snow Leopard backup, but your time may be spent there for no need or you could be "committing" your time later to debugging things.

It really depends on how much changes you made and the details of what changed file by file, preference by preference. Some will not notice a thing, you'll want a little luck on your side if you try it, but perhaps not even need it.

Let us know how it goes!