Hi David,
Coincidentally, I just posted
this addressing that exact issue.
The answer is: yes, this will be supported quite soon. It was supposed to be in release 0.8, but didn't quite make it. It's finished now, just needs a little more testing (let me know if you want to try the beta).
This falls under the category of a previously-requested feature known as "Auto-populate arbitrary fields". People have wanted PasswordMaker to auto-populate fields like PIN on their bank site, which might require 3 fields to login (username, pw, PIN). The way "Auto-populate arbitrary fields" works is you provide a name/value pair to PasswordMaker. The name is the HTML field name; value is what to place in the field. So that you don't have to read HTML, you can simply click on the field in question (with PasswordMaker open) and the field name/value gets pre-populated for you in PasswordMaker.
In this context, I hope you can see how "password" is just another arbitrary field. If you use "Auto-populate arbitrary fields" and select the password field as an arbitrary field, you can then provide any value you like for auto-population.
Of course, as you pointed out, this value is saved to disk (the passwordmaker.rdf file). However, the values are encryped on disk if that's any consolation :)
Regards,
Eric