OTA update limitations with custom ROM?
It would not seem to make sense to flash a ROM for a customized android TV launcher - all you need is something to automatically start your app when you boot - maybe something like https://play.google.com/store/apps/details?id=com.autostart

If you build your own ROM you can make it do whatever you want - including check for updates on your own server - this is probably not trivial, and anyway, where would the upgrades come from.

If you flash your ROM you will no longer get OTA updates as you are no longer running the software associated with the OTA updates.