Is there an accepted metric for "the value of the dollar"? Is there a reasonably well-established scheme for estimating the value of the dollar in international trade, somehow relating it to the "average" of other currencies or to some currency-independent metric?  Something that could be used when researching the relationship between fiscal policies and international trade.

(I would consider using the Euro, but I'd like numbers going back to the 70s, at least, and obviously the Euro doesn't work for that.  In addition, it is perhaps a bit destabilized by, eg, the Greek economic crisis.)
It sounds like you want the Trade-Weighted Dollar Index, which is available from 1973 on for major currencies. It's the value of the dollar against a basket of other currencies, weighted by their share of US trade.