I've created rules that reference request tokens, but I'm trying to create a validation that references a request token. Reason being, as I'm sure you're aware you can't key a rule off of another rule. This may be a way around one of PPM's limitations.
My guess from what you're saying is that you want the field to auto-populate based on another request token. I think the best you can do is populate the results of an auto-complete using a validation that contains other request tokens such as this example.
select '[REQD.VP.TEST1]', '[REQD.VP.TEST1]' from dual UNION select '[REQD.VP.TEST2]', '[REQD.VP.TEST2]' from dual
If you give a little more detail about what it is you're trying to accomplish, we can probably come up with a workaround.