@Addison
I'm successfully using an N900 BL-5J battery in my N810. The BL-5J has the same type of terminal connectors as the BP-4L with the same +/-/BSI terminal arrangement, but it is a 1/4" smaller than the BP-4L in both length and width so you have to use some type of filler (like blue-tack or an L-shaped piece of corrugated cardboard) to keep the battery in place.

For extreme battery life, you can try out some of the battery mods inspired by Dr_Frost_DK's research...since the N810 is bigger and has no camera on the back, you can probably easily fit an additional two batteries on the outside of the battery cover instead of just one.

As for the N800 BP-5L, I'm not sure what recently manufactured battery models are good substitutes. I think it should be possible to chop the top bits (the tiny circuit board and battery terminals, basically the non battery portion) off from an expired BP-5L and a new BL-5J and to then solder the BP-5L top bit to the newer lithium ion from the BL-5J, but I'm really crap with electronics so can someone chime in and say whether this is a sane idea?