It's for sure something I though about but difficult to implement in a safe manner. Basically, your displayed hand is continuously synchronized with the server as you discard and draw tiles. This implies having some knowledge of what is where and relying on that knowledge. I'd be worried that it has the potential to introduce many new bugs. I might have a look at some point regarding feasibility, though.
In any case, free dragging/dropping is not really an option due to the layout restrictions of BGA and the desired cross-platform support but more constrained solutions (like the 2nd one you mention) are doable at the UI level of course.

I found a way of doing this using buttons (swap + resort). It's a bit clumsy but I need/want to stay away from overloading the standard left-mouse click used for discarding tiles with dragging/dropping. If I did that, I might force players who want to use this feature to enable confirmation of discarded tiles since accidentally discarding a tile (likely especially on a touch device) from an attempted drag action would be very annoying. Having tried it out a bit, I think that one of the unexpected benefits is simply having something to do (which some players might enjoy).

I should be able to make it available relatively soon.

gilthe

I have incorporated this into the latest push, so I am closing this as implemented.