Parts came early so I've got some mod updates.
Regarding stereo output:
The good news on this front is that the DS-Lite speakers are exactly
the correct size (and impedance). They slot in where the existing speaker goes perfectly. These must be a standard size or something. Now - there is also plenty of room for a left speaker beneath the d-pad. Some minor trimming of the select-button plastic is necessary, but it fits without issue and the button works just as it did before.
Disappointingly, it would appear that the solder pads I identified are not carrying a sound channel. I spent a few hours trying to identify some points that would
carry a left-channel signal, but it would appear that the necessary components are not populated on the board (likely a cost-saving measure). Fortunately, as bartimeaus suggested, this can be solved by installing a small, cheap stereo amp, which should be quite easy (for someone actually considering these kinds of mods).
I went ahead and installed the left speaker on my device since I was in there and temporarily wired it up to the pads for the existing sound channel (so essentially it's dual-mono until the stereo amp I ordered arrives).
The keen-eyed among you might also spot another 'mod' (and my terrible, shameful drill-holes for the left speaker lol). I carefully applied black electrical tape around the edges of the display. This eliminates backlight bleed and just looks a lot cleaner (especially coupled with the rs-90 d-pad and the all black DS-Lite speakers).
Single-cell gigantic battery:
Inspired by steward's mod, I tried to find the largest single-cell I could find to cram into the rs-97. It took A LOT of doing, but I managed it (just barely). This is a 3.7v 4000mah "5044147" cell. I knew it would be tight, but I didn't think it would be THIS tight:
The shell needs heavy, heavy trimming to accommodate it, and multiple components will need to be removed/relocated on the mainboard. I removed the TV-out jack (never going to use it anyway) and moved the power button to where it was formerly located (since that stuck up too far in the wrong place). The old battery connector is a given, The EXT port may as well go (not in the way but I plan on relocating the ext-sd there in the future). Unfortunately the headphone jack had to go (though I will be relocating it later on as well) and the faux-gba cart had to be hollowed out.
But, It all came together in the end:I'll answer any questions that haven't been answered already, but please, if you must quote this post, highlight everything and write --snip-- instead of quoting the full thing.
The stereo amp as well as a QI wireless adapter are on the way, I'll update again once I get those installed.
Cheers and happy modding everyone!