Those dampening feet might be perfect, and I think i know someone who has a bunch. they're also solid rubber, no?
As for the doorstops, they'd work if they were long enough, otherwise the angle would be off. the only doorstops at my house are way too short to work correctly.
my chair's already up all the way, and locked so i can't lean back. that's the way I usually sit when typing.
When I do lean back (usually at night), that's when a good keyboard angle can come in handy, i've found.