Mapping the paddle buttons to meta keys was such a good idea that Apple used it on the Apple //e with the open-Apple and closed-Apple (eventually option) keys. Was that a common thing for extended keyboards in the Apple ][ era?

It was standard practice to wire the shift key to paddle button zero to get upper/lower case when you had an 80-column card. But I used full ASCII keyboard that supported upper and lower case, and had two extra buttons that were easy to wire up to paddle buttons and use as modifiers.

"For lowercase input, since it was not possible to detect whether the keyboard's Shift keys were in use, a modification called the "one-wire shift key mod" connected the Shift key to one of the pins on the motherboard's paddle connector. Compatible applications, including nearly all word processors, could then detect whether the shift key was being pressed. This modification, however, involved adding wires inside the Apple II, and was therefore only popular among hobbyists. For this reason, most applications that could support lower-case letters could also use the ESC key as a substitute lowercase toggle if the "shift key mod" was not installed." -- https://en.wikipedia.org/wiki/Apple_II_Plus#Substitute_lower...