How can they stop you?

In general, you can write a UI that will invoke external commands such as the flasher3.5 executable, read it's output, and parse it to give the user some info with a nice GUI.

It's perfectly doable, but in order to JUST-WORK, you need to know the possible output from the external application, and you need to do it for every OS that you want to support.
Not very hard, but takes time.

This would be rather easy in python. Neccessary libraries: subprocess, PyQt, tarfile and mechanize (to navigate the EULA screen, assuming you want the flasher automatically downloaded)

<snip> Code available in repository

Then from a PyQt class you would just need to call flash() and parse the output. Hmm...kind of fun. I guess I'll start a gitorious project for this..

done: http://gitorious.org/qflasher

And no, I'm not packaging this up and making a deb out of it. Anyone else who feels free to do so, knock yourself out :-)