Another bottlejack/hydraulics Q

There have been discussions in RCM about modifying bottle jacks to be operated in the upside-down position in a press frame (with a separate hand pump), and I think someone had pointed to a website that had a parts breakdown and pictures showing the required parts.
Trouble is, I don't have any idea how long ago that was, but a search of the RCM archives over the last 2 years would likely bring up the discussion(s).
Since you have a power pack, you'd want to substitute the existing hand pump piston with a fitting for pressure/fluid flow in (if the base casting is suitable for this type of modification).
You'd also want a reliable overload bypass (already exists in nearly all jacks), and a release valve to allow the fluid to return to the power pack to allow the ram to return to the down position. If the fluid can't return to the power pack, the fill plug will likely blow out when the jack reservoir gets full, maybe. Some jacks utilize external springs to withdraw the ram from the extended position when released.
You shouldn't only rely upon the release valve in the power pack, because the hose could develop a leak and drop the jack.
The overload bypass is critical, so you don't end up blowing seals, but then your personal life is up to you.
What you choose to do in the privacy of your own shop is your business, but hopefully you have a more rewarding, more tasteful source of income.

Given that you can get a set of them already fitted for an external hydraulic supply from HF for about $100, why bother?

I put a 12 ton air/hydraulic on my arbor press last year. Its awesome, but I need to remove the little holder clip so that it can not be locked in the continuous position. Oops.
$79.95 from HF if I recall. My little portable compressor runs it just fine. I also pop it out of the press and use it sometimes when I need more than one jack on the floor. For heavier stuff like the front end of a truck it seems to struggle a little, and I know that is nowhere near the 12 ton claimed capacity. Front end of one of my 3/4 ton pickups is a lot lighter than that. I have actually considered picking up a couple more of these jacks or similar just to have them for stuff like this though. Its handy to just hit the air instead of trying to find an angle to work the handle on my floor jack. I've also considered an air operated floor jack.
Anyway, they are pretty affordable. If you spent the time working on something instead you could easily spend that earned money on an air operated hydraulic jack and have money left over. IMO.

I know this isn't an answer to the question here but it may be useful.
One time I made an IV bottle type of fluid reservoir for a hydraulic jack that allowed the jack to be used in any position. I used a short piece of PVC pipe and fitted it with a tube fitting. I also made a new fill plug for the jack and drilled it and soldered a tubing fitting on the plug. I attached a piece of tubing between the PVC pipe reservoir and the jack, added hydraulic jack oil and then (the hard part) did my best to get all the air out of the jack. The idea is to remove the air and completely fill the hydraulic jacks internal reservoir so that air can't get to the pump. The external reservoir then supplied the oil for the jack's ram displacement.
I hung my reservoir on the wall near the press I was using it on but it could be designed to set on the floor. I think I could have improved on removing the air from the jack with a brake bleeder vacuum pump kit but I didn't have one at the time.
RogerN