The problem with adding seat belts is that it is (obviously) a structural mod. The attachment points need to be suitably robust, and most planes don't have "structure" up there.
A 337 is a Major Alteration approval form. If you wanted to do a major structural mod on an N-reg you would likely need a DER to generate some analysis and produce an 8110-3. This is likely to cost you at least 4 figures.
I was once quoted $10k for a DER package for a relatively trivial avionics installation (probably about 2 days' work for the DER) which could have been done (and was done) for nothing by getting an FSDO to approve the 337 for the installation beforehand. But major structural mods to the airframe are a different thing and the FAA is very picky about those.
Last edited by peterh337; 14th August 2012 at 07:03.