Yeah, OP_TRUE and the like are the same as a widely-known private key. I don't see what advantage you get from treating them different at the protocol level, though I suppose a smart client could attempt to inform the user if a payment they received was more likely to be attempted to get double-spent than average, but I'm not sure the user would or should do anything different in that case anyway.
No current client will tell the user "hey, I found an anyone-can-spend" nor list that in their balance.
Clients pattern-match output scripts, and only "see" ones with scripts they recognize.
All other transactions are simply invisible without special software to find them and spend them.
). However, any transactions relaying on "anyone-can-take" outputs, such as