My real question is how do you block a party without Crono from doing the Zeal events?On a technical level, it's pretty easy. Just set up an event that checks to see if Crono is in the party, before going to Enhasa, then lock him in, during.
Storywise, though? There's technically no reason to ever drag Crono along on any of this mess... or, really, any of the events of the game. But, between Ayla and Marle, I'm sure some story could be written to where they absolutely demand his presence.
Like, for example, let's say Ayla wanted him to help attack the Tyrano Lair (because, she thinks he's the strongest of her strong friends) and refused to fly the Dactyl, without him. That seems like the most logical point to lock him in, to me, without making things seem too obvious or cliché.
Alternately, the "lock point" could be set to after finding the Epoch. But, Janus' vague proclamation would have to be edited to accommodate, if Crono isn't present, at the time.
There's a whole lotta ifs-and-or-buts to consider, when changing something that significant. It's, honestly, kind of a mess. But, I reckon that we can help, if Ben needs it!