The Transform panel extension by Trevor McCauley may be helpful to you here:
It allows you to set an origin point; I can't recall if it works in conjunction with Numeric Transform or not, but it may allow you to transform numerically within the panel itself.
I have never used any FW extension before and am not familiar with its installation. Could you test whether it works for numeric scaling?
I had this installed on FW8, but not my current version (haven't gotten around to it yet).
It turns out that it doesn't work with the Numeric Transform command, but the panel itself will allow you to transform numerically if you "dial in" the right settings. For instance, if you want to scale by percentage, you must click "Relative" and set the Units to %.
This is such a good extension, I don't know why it never became an official component. (It does seem kind of old, though. Hopefully, it still works fine.) This article offers a couple panel extension recommendations:
To install an extension, use the Adobe Extension Manager (in your Applications folder).
I agree with you that this extension is really such an essential thing that I wish FW to natively come with it. It brings to FW the ability to choose transform origin that illustrator and many other tools have.
Thanks very much for help.