Originally Posted by xen
Just a word of caution on the sand in the seatpost - most tubes in a bicycle frame are not sealed off from one another. Bike manufacturers purposefully drill holes in the head tube, seat tube, and bottom bracket shell where they meet the other tubes (I have no idea why, but they do). If you dump sand into the seatpost, it could potentially fill up the bottom bracket area that houses the bearings for the cranks. If you have a cartridge bottom bracket then you wont have much of a problem, but if you have an older style bottom bracket, which is very likely on a bike from the 90's, then you will be dumping sand directly into your bearings. You may want to check it out before you fill it up.
I was thinking the same thing, but with another potential complication. If abrasive sand plus vibration causes the sand to work its way into the bottom bracket threads, it could make the BB darned near impossible to remove. Sand and machinery are just a bad combination all the way around. It works with handlebars because their openings communicate only to the outside world, and not to the bike's internal workings.
If the OP wants to ballast the frame or any other part to damp vibration, it would be a good idea to add ballast in the form of relatively large lead shot-- I'm thinking #4 buckshot-- to make the ballast easy to remove later and unlikely to leave lingering abrasive debris. Stay away from hard abrasive sand and materials that could absorb and retain water (because this is a recipe for truly horrible corrosion).
Vibration damping is one of those places where a little might be a good thing, but that does not mean a lot of it will be better. A normal bike frame is a pretty stiff truss, as opposed to normal handlebars which are cantilevers. The poorly supported cantilever is much more susceptible to resonant vibration than the truss.