Ok I get where your going, but these are commercial style kegs that have been rejigged, The idea is you ferment @ what ever normal temp* @ say 10-30psi with a spunding valve (do some reading on pressure fermenting this ain't the thread for that), when you hit 10 points above fg or there abouts, remove the spunding and let it rise in pressure, then when you cold crash the excess co2 will be absorbed into the beer, carbonating the beer. If your going to ferment with a blow off and then seal, the vacuum created won't harm the vessel, but I would suggest hitting with 10psi of co2, if won't harm the brew.
* when fermenting under pressure temps can be higher due to the pressure inhibiting ester formation (have an ale on tap that saw 40c in summer drinking quite well. Note this is extreme I'd prefer 25c as a ceiling)