I do have this on my suggestion board, and I did start to look into it. There are some complications to consider, like for example, if you enter a pvp zone, and then exit it (which starts the delay timer) and then you enter a different forced zone (say it forces you into "hardcore" mode or something) what happens? I would say, that surely it needs to force you into hardcore mode (even if you're on the delay) and then when you leave the hardcore zone, I guess it would just reset the delay timer, so you would stay in hardcore mode while on the delay.
Another thing to consider is I know people will immediately want to be able to control the delay for each mode, or even for each specific zone. Because they might have an area that is forced pve that they dont want a delay for, but they might have another area that is forced pvp that they do want a delay. Combining this with the first scenario, you could potentially have an issue where people can avoid the pvp delay by traveling to a forced pve area (that doesnt have a delay) and then when they leave that area it would get rid of their pvp delay.
So still a little bit of thinking to do on this one, happy to hear your thoughts on these issues as well.