I can think this through, but I can't program it because I don't know enough.

1) the browser treats everything that it receives through CDML as though it had been typed in the page's file.

2) this can break JS/CDML scripts if the CDML includes returns or other delimiters (drag)

3) on the other hand, it means that FM can send any JS (anything in text at all, really--and since HTML is all text...) to the browser

4) and JS can send script commands back to FM (I assume this, may be correct)

5) so you could use your browser's ability to do JS (including GREP and arbitrarily-large programs) even when you're not really serving web pages

6) if they could be tightly integrated, we would finally have a real language

Is this worth pursuing?

1) the browser treats everything that it receives through CDML as though it had been typed in the page's file.

Yes

2) this can break JS/CDML scripts if the CDML includes returns or other delimiters (drag)

Yes, this is a pain. So you do have to use tags such as [FMP-Field:myfield,break] etc.

3) on the other hand, it means that FM can send any JS (anything in text at all, really--and since HTML is all text...) to the browser

Yes, you can use the [FMP-If] tags to decide which JS is being delivered to the browser. You can even have it stored in a field.

4) and JS can send script commands back to FM (I assume this, may be correct)

Yes

5) so you could use your browser's ability to do JS (including GREP and arbitrarily-large programs) even when you're not really serving web pages

Yes, this is powerful. It takes some of the processing away from the database server.

6) if they could be tightly integrated, we would finally have a real language

Is this worth pursuing?

Yes