An interesting case study. Is any captain permitted to prohibit an AIPA member from engaging in PIA? If it were PIA regarding uniform, tour of duty, etc, then we'd be rightly up in arms about it and the crew member concerned may be in a spot of bother prohibiting a subordinate from taking part. So why is the PA different?
Keg, if there were something in company policy or even in the current EA that gave the CPT discretion, then I'd say yes it is very much possible.
What it boils down to is if a "workplace right" (ie the right to take PIA) has been impeded by actions that are in any way contrary to "normal" operating procedures.
There is some recent case law on this, I'll try and dig it up.