My thoughts were to decode IR buttons with some sort of central controller or computer, then send WiFi, RS-232, etc. as needed, from the controller.

Beyond the basics of Volume/Play/Pause, you'll have a difficult time dealing with Sonos exclusively using hard buttons.

As you are discovering, very few companies can afford to bring out a custom handheld similar to your Pronto. For a similar or much lower price one can purchase a full pad or phone that can perform many more functions. One of the reasons these pads and phones can be so cost effective is because they don't offer many hard buttons. Most of the public will gasp and become angry at the price of a Pronto style remote compared to the cost of their pad/phone that was "free" when included as part of a cell package.

Remotes that do offer some buttons are bundled with controllers and professional programming.

I would prefer more hard buttons on the handhelds too, but these are not in the DIY scope at present.

I can't, but you might be able to make do with a Harmony.