Use Dark Archons for your buy units area. You can set the Mcing rate to cost like 250 and whenever the units u mc are on your spawn you just move them to a spawn point and make another unit for your DA to mc.
I'd say it would work well for heroes (just make MC cost 250)
This has actually been done many times to purchase heroes and generally make choices. The best map that makes use of it is Shaded Def (though the map is not that well known).
I suppose it's an interesting idea, but really I think it would only be useful if you really need to save on triggers. Normally I'd just use the standard system of units walking onto beacons.
beacons are noob, you can walk onto the wrong one(happens many a time), they take up more space, and they require more units(civs) to use.
That's why its all about disabled beacons.
As far as a newer heroes system goes, you could use a Dropship system as a menu (like what Ultimo and I did in Transport D). The only issue is that its really confusing for a lot of people.
MC select is my favorite way. I've grown to hate beacons. They're slow to make selections with, use up locations (yes I know you can recycle them but that's a pain by itself), and they're overused. Plus MC selects are good for making rapid multiple selections (second only to factory selects), and you can quickly pick from as many options as you can fit within the MC range of a DA (factories and dropships have a fixed limit of options, by comparisons).
This has already been done in all those old Protect the Queen maps and Mind Control Defence
And Trigger Happy Defense by tux. It worked out really nice.