Wait, having a repeatable quest as an instance to earn the reputation is a good option, just not that the workbench would be actually be there in the instance, otherwise it would lose the whole point of having them tied to Gunrunners or any kind of faction.
Actually possible solutions are rather easy to imagine, you just need some imagination
for that. E.g. gunrunners wouldn't allow anyone unless they were proven to be worthy
- with entrance to small "instanced dungeon" undergrounds in the gunrunner shop and
exit on the island. You could go back by simply talking to the soldier on the bridge.
Although I suppose logging off on island should kick you to WM, or you could bring more
mats with non-crafter.
Alternative could be "Hell no, we don't allow strangers to use our advanced equipment,
but you know what, in bunker below there is one workbench we don't use - if you manage
to survive there, then go ahead.".