First version of an extension is available on github, rkl099/Appinventor-SerialOTG
EDIT: Here is the link: https://github.com/rkl099/AppInventor-SerialOTG
It is compatible with the built-in component Serial, extended with settings for parity, rts, dtr and refactored so it can be used with AI Companion. It has been tested with CdcAcm, PL2303, PL2303HX, FTDI, CP210x, CH341, CH341 (“fake”) and several android devices. I hope this will solve some problems discussed in the forum and make new applications possible.
Please return if you have any problems.