There was a previous formal request for something along these lines here:
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As I commented on that issue, there was previous work to add the same to Arduino IDE 1.x, which was well received by the developers and community, but unfortunately came after the active development on that IDE series had ended. So I do think there is hope we will see something like this eventually.
As for whether it would be possible for the users to achieve a similar result without modifying the Arduino IDE source code, it could be done easily for a subset of the boards of one platform (e.g., "Arduino AVR Boards", but I don't see a way to combine boards from several architectures (e.g., AVR, ESP8266) into a single boards platform.
There is technical information on the Arduino boards platform framework here in case you are interested in the boring details: