Is it possible to use app browser to load from a local repository on a flash drive (sda1)?
yeah, any kind of drive. you can use appbrowser (load local) to pick any tce or tcz extension from any filesystem you've mounted, including mounted iso files. it's also possible to do it from the command line using sudo /usr/bin/tce-load. either way, you need to have the deps (the other needed extensions) mounted as well. to know what deps there are, there is a .dep file for each extension that lists them.
appbrowser and tce-load only automatically load deps when you are downloading. to simulate this you'd need thehatsrules' instructions.
for many uses (it only depends on how automatic you want it to be) you only need the extensions (saved on whatever) and appbrowser, you can use that anytime. but the easiest way to load them is just put them in your tce folder and reboot.