Weedeater line or small rubber tubing. Adds weight without effecting spine.
Have you bareshafted after doing this? I'm with Keith, in my experience it does affect dynamic spine making them stiffer (not static though).
There are plenty of options, vinyl tubing, irrigation tubing, wooden dowels (noisy though!), sand......but they do need to be full length as you suggest and it sucks trying to get stuff out if you've got a nock bushing too!!!! Wraps might be an option depending on how much weight you want to add.
I'm sure there is some relationship between adding weight at the tip and the tail that doesn't affect dynamic spine, but what % it would require may take some shooting to work out. The relationship may not be linear either i.e. if you have to add say 5gr in the nock to offset 25gr up front, at 50gr up front it may not be 10gr. It may be possible to JB weld a nut into the nock so that you can then cut lengths of threaded rod/bolt to tune each arrow weight and just use heavier points/inserts.