What follows will not solve all your troubles... not even all your kbd-related troubles... but may help. This process will allow you to strike F-keys with effect and get some refractory formspecs to accept kbd input.
- Install Hacker's Keyboard. This tool is useful far beyond the game. You do not need to uninstall your other kbds.
- Despite its name, HK contains no magic kracking abilities. It's just a full-featured kbd.
- Some apps allow you a button with which you can switch kbds. Minetest is not among these yet. Find one and switch to HK. This workaround only works if HK is active.
- Open the HK app and drill through HK's settings until you find "Permanent notification." Turn it on.
- I'm deliberately not telling you how to find this setting. Any instructions would be confusingly verbose... I think. Go look.
- Enter Minetest. When the game expects you to type, HK will pop up. Enjoy many-keyed goodness.
- When the game does not expect you to type, find the permanent notification in the bar. The bar is hidden by the game so swipe down. Beware vertigo. Tap the notification and out rolls the kbd.
- Now you can type anyway. You can strike F-keys if you like.
Other formspecs assume you'll hit a carriage return. These don't work for the Android player. Return is relabeled "Done" and just closes the kbd. The string you typed is now in the formspec but not to stay. If you now close the formspec you will have done nothing.
So yes. Open the difficult formspec, tap the field, enter text or digits, hit Done. Then immediately swipe, tap HK perm note, and bring up the kbd a second time. Now hit Return.
HK does not solve all input problems. But this workaround does address some common cases. Good Luck!
See also: WikiHow