I've read that some guys did it successfully ...
If we check this reference web page, it seems to be almost impossible:
https://kan.org/6100/cpu.html
You guys got me interested enough to pull the 6100/60 out from under a pile of binders and other crap. So far I only see physical limitations to the G3/HPV combo due to cubic interference with the standard 3.5" HDD mentioned in the docs. One look when I popped the lid had me remembering plans to use one of my 2.5" SCA drives a decade ago. No problem in terms of cubic there, but SD developments over the years negate power/cooling budget concerns I had at the time related to use of the little server drive.
Clock Chipping
Including stats on how well people did overclocking.
Overclocking stats seem to be related to OEM CPU constraints. No concerns there when using a G3 card AFAIK? The 601 sleeping on the logic board doesn't enter into the equation at all with the G3 card present.
Power/cooling remains an issue, but that doesn't look to be much of a problem given 3D printing developments since I last looked into this. Plenty of cubic is available behind the blank section of the backplane plate adjacent to the HPV's DA15 connector. A printed replacement for that plate incorporating mounting points for a centrifugal blower (squirrel cage) ducted for exit at the rear takes strain off the OEM fan, improving cooling significantly. Ducting on the intake side pulling air straight across the G3 Card's heat sink directly from the intake vents underneath the HDD bay should make things much better on the cooling front.
Centrifugal blower tech has come a long way in terms of efficiency and noise dampening. They're in everything now including your laptop today. No worries there. The original SE's cooling utilized a bypass fan, not a "Squirrel Cage" design, so don't bring up that example.
In terms of SD card and air dam induced by the dog legged cable run to CD/HD and above mentioned interference with the OEM 3.5: HDD, we can do a lot better I think. Looking into a carrier board for an SD solution that plugs directly into the internal SCSI connector that provides for a straight, flat cable passthru run to the CD connector makes sense to me. Eliminating SCSI cable to the HDD bay will improve airflow.
Implementing a sensor on the G3 heat sink with speed control circuit for the rear blower mod seems like a nice project as would be coming up with the printed bracket. Such would be implemented on the SD carrier/passthru PCB. I don't do the modeling/printing or electronics sides of these things, I just come up with crazy ideas.
Dunno, I think we have solutions available now that render HPV fitment problems mentioned in the docs moot, even improving upon power/cooling budget limitations of the stock 6100.