Are you going to have it connected to serve? I'm just trying to picture the set up.
Because co2 dissolves easier in cold solution, there are two things I'd consider doing. The easiest would be to prime the keg just as you would bottles and keep it at room temperature until carbonated. That would give you a consistent carbonation, but not force carbing.
The other thing that might work is to chill the keg. Then, add the co2 (at a high pressure like 30 psi) and gently rock the keg on your lap until the co2 stops flowing. Stick it back in the fridge and see if it's too foamy or undercarbed the next day. If it's still undercarbed, repeat- if it's overcarbed, purge and put it back in the fridge. That would definitely be an inexact way to do it, but it might work ok.
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