It is possible to seal taps for high pressure coolant thru operations. The success or failure is dependent on the tap length and flute lengths ground into the tap.
From my research tap manufacturer's are anything but consistent. There are hundreds of shank and square combinations, relief grind lengths very inconsistent batch to batch this makes sealing taps more difficult than it needs to be.
Long tap shanks is the best condition for using coolant caps and sealing for high pressure coolant up to 750psi.