it looks good, having both, a tce directory and two iso images (CorePlus-4.3.1.iso and
CorePlus-4.4rc3.iso) on the usb stick, I added two options (i.e. waitusb=5 and
iso=sdb1/TinyCore/CorePlus-4.4rc3.iso) to the command line. During boot, it tried to
load (first) my extensions from the tce directory (with problems, they are for 4.2.1) and
then the embedded extensions from CorePlus-4.4rc3.iso.
I guess this behaviour is expected, I'll try to rename the extension directory for a check.