I remember back in the day doing a tualatin pin mod, but don't remember much of the details... it was over 10 years ago. I did it on a slocket adapter though, to be used in a slot1 motherboard, BX chipset (Abit BH6). What I did, to keep compatibility with coppermines, was insert small plastic tubes (basically little pieces of insulator stripped off copper wire) on the pins of the CPU itself, then enlarged a bit the corresponding holes in the socket so the pins with insulator would fit. This way I could still remove the tually and insert a coppermine if I so wanted. Worked quite well IIRC.

Not sure, but both the Upgradeware Slot-T and Powerleap PL-iP3/T adapters are Coppermine + Tualatin core compatible, without changing any jumpers.
I would not want to mod a motherboard this way, a cheap slotket maybe. A Lin-Lin adapter would do exactly what you want, have you considered these? Or the pre-modded Tualatins from ebay?

Hi Ace i made this mod some years ago
http://pipux.net/index.php?id=15
Only problem is that it seams to have problems with tualatin@1,475-1,45 not with 1,5v
I made it on my asus cusl2 (same your board) with one Celeron 1.3 A1@1.5v; I insulated only those 3 pins and still work like a rock 😀

Good to know the pin mod works on the CUSL2, but I still have no answer for my main question. Can you still use Coppermine CPUs on a pin-modded Socket 370 board or do those boards become limited to Tualatin CPUs only?

Well since the modding is done actually on the CPU not the motherboard, I don't see why you couldn't later plug a coppermine. Just make sure when you enlarge the pin holes in the socket that you don't damage the contacts underneath.

Or if you don't want to deal with enlarging holes in the socket, another method I remember hearing about back in the day for isolating those 3 pins was to coat them with nail polish. That would probably allow the CPU to slide in without tampering with the mobo at all.

Or just cut the pins that are just going to be isolated anyway? The one pin that cant be fully cut can perhaps be shortened?

A 1.2 GHz Tualatin is not worth much at all in my book compared to an Asus CUS2 motherboard which I never would risk damaging by destroying the socket.

At this rate, I might as well just use the Tualatin Pentium III on the TUV4X and max out the board as much as I possibly can. I will say the 512MB memory limit of the i815 chipset is highly unappealing to me since I plan on doing a Windows 98 SE/Windows XP dual-boot (unless I can figure out how to do netplay with X-Wing vs. TIE Fighter under Windows 98, which I have had no luck with so far) and the VIA chipset on the TUV4X can take up to 1.5GB of RAM.

A bit off-topic, but still somewhat relevant... between a GeForce 6200 and a GeForce FX5600, both with 256MB of RAM and both AGP, which one would actually perform better? Currently, I'm testing with Star Wars Rogue Squadron 3D, which, on my K6-III+ build running a Voodoo 3 3000 AGP, is the game that experience the most amount of framerate drops on that hardware setup. On the TUV4X with a 1.2GHz Tualatin Pentium III, 512MB of RAM and a GeForce 6200, the game runs exactly as it does on the K6-III+ build with 256MB of RAM, the CPU overclocked to 600MHz and the Voodoo 3 3000 AGP running in Glide (I don't use Direct3D with this game because the framerate takes a plunge when I do). Is the board just a poor performer or is the GeForce 6200 that underpowered? How would the GeForce FX5600 fare in comparsion, and for that matter, what about equivalent ATi cards? I usually don't use ATi hardware because I can't figure out how to upscale the video signal over DVI to 1920x1080 (I can do this with relative ease on the GeForce FX5600 and GeForce 6200 - if you're wondering why this is necessary, it's because my Elgato Game Capture HD60 refuses to record DVI inputs unless they're 1280x720 or 1920x1080), but if ATi cards perform better (I think I have a Radeon 9800 somewhere) and the DVI output can be properly upscaled to 1920x1080, I might just go with ATi. Otherwise, what would be good options on the nVidia side?