I'd imagine, you could upgrade the battery with any 3.7v lipo cell. If you don't mind a big cumbersome, heavy mod, you could theoretically add a 10000mah or more 1s, or 1sxp lipo pack, and devise a way to connect it to the original leads. The stock battery is 1250mah, so well, you do the math .

Add a few caps of the right rating, and you could swap out that pack, to a ac rectified dc power source using a switch without rebooting. Or ideal diode mosfets could allow you to parallel two power sources (ac/dc rectifier (adapter)) and a battery. I use something liket that for a dc atx power supply for an itx amd 5600k fm2 system, that switches fine from a smps and a floating 12v 18ah sla in the event of a power failure. The power supply senses when the battery's voltage gets too low, and gracefully shuts down the system.

Just for the love of god, use a protection pcb on such high capacity diy lipo systems, and fuse it to help you sleep at night.

Really, all you need i would imagine, is to get a 3.7v source to the two +/- pins where the battery goes. Your source of power is endless, solar, mechanical>electrical (hand-crank & big caps lol?), big or small batterys of various voltage with a sepic regulator, and the right protection from overdischarge, an ac/dc adapter with a variable voltage output, or use a sepic to get the right voltage.

Just fuse any custom mods you do, pcb protect lipos/any rechargable battery from shorts/overdischarge, and don't turn your n900 into a charred several hundred dollar paper weight.

Personally, i carry around a 2s 7.4v 1800mah lipo, with a 5v ubec regulator attached to a female usb lead, with the data pins shorted. Get a few charges out of it per pack charge, and the same goes for my nook color and a 14.8v 5000mah pack i have (sucker draws 25 freaking watts, or 5v/5a when the battery is low and its under heavy load). But thats almost a half kilo pack. The ubecs are ~25g, and you can use any 1-6s pack (lipo,life,li-ion). For ~$30 you can build one of these, and draw up to 5a of current from a $10 ubec. Thats like 5 n900's , or a tablet and other 5v devices depending on the current draw of the tablet. Capacitors can help prevent voltage drop under high current spikes.