You could drill an access hole and plug it afterwards, the main nut and the base would cover it. I know it sound extreme but it might work as a last resort.
You could always thread the hole and put a set screw in it and call it an access port.
I'd considered it, I even have machine shop access. However I agree, I will only drill it as a last resort. I have also tried a screw Jack inside the vise, which didn't work either. And of course using a copper hammer beating on it inside the vise didn't do anything. I'd be more inclined to drill and tap the top of the pin itself, and try using a slide hammer.
I'm jealous your pin came right out. This is my 3rd swivel jaw vise and they've ALL had stuck pins.
I don't think in almost a half century of trying I've EVER had penetrating fluid loosen off a corroded in bolt or pin. Doesn't stop me from continuing to try, but they say that's the definition of insanity.......
Tried ATF and acetone, PB-blaster, kroil, WD40, and a host of others.
My method thus far has been to fill the swivel pin hole inside the vise with the body upside down, let it sit, check a few days later. The level of the fluid goes down over time, I think it is just going into the void between the jaw and body though. This makes me think the pin is seized in the swivel jaw itself, and I should exercise caution to not break it.
This vise is a long term project. Despite looking like it didn't see much use, the slide required filing and stoning to slide freely. The swivel jaw is stuck. The spindle was stuck due to a burr on the c-clip groove, but a BFH got that out. Oh and the jaw screws all required an impact to get them out, but one is still stuck. The pad with both screws out flatly refuses to budge. Rant over lol