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Is that a bad idea to consider hand-editing it?
Not at all. You'll want to make sure that you go through your HTML options and configure the editor to do what you want it to do (tabs, spaces, etc.). If you don't, FP is going to reformat (based on its default settings) what you open unless of course you've already been into the options and have compensated for that.
In fact FP is going to give you some features that other programs won't while working in HTML view. And, Expression Web (FP's replacement) takes it even a step further. :)
FrontPage HTML within NotePad or some other text editor? Would you know of any restrictions or drawbacks to doing that?
No but...
If the site utilizes any of FPs functionality like buttons, navigation, themes, etc. you won't be able to edit in anything other than FP.
If the site is free of those nasty little add ins, then you should be fine updating in Notepad. Keep this in mind though. FrontPage remembers everything. Using FPs HTML view is just like using Notepad but on Steroids. If you update a link while in HTML view, its going to send that update through the entire site. If you do that outside of the FP interface, the change will not propagate.