Would pastes made from vinegar and seeds other than mustard seeds be palatable?
Palatable is a very vague term. I think that some seed and vinegar preparations would have promise (sesame seeds and rice vinegar, perhaps lightly sweetened?), but others would be horrible (I can't imagine a caraway &amp; vinegar paste being good for most things). Even a condiment that would normally be gross (the caraway one mentioned) could be good in the right applications, for instance in a sandwich tailored specially to use it. 

I'd think this would be a good instance to just try out some formulations based on known flavor combinations and see how they work for you.