How do I specify a semi-transparent text box in Framemaker?

Technically speaking, postscript is still an opaque colour model. However, there now is a pdfmark operator that allows Distiller to create transparency from postscript in PDF versions 1.6 (Acrobat 7.0) and greater. This is the SetTransparency pdfmark. So, using postscript text frames in FM does allow one to sneak in the transparency effect, but one has to also manually enable the transparency in the PDF joboptions file [this is not accessible through the Distiller edit interface] by setting "  /AllowTransparency true".

In the example below, I've created a postscript text frame in the Aframe with the image. The postscript code specifies a green rectangle with a 50% transparent orange rectangle partly covering it. Note the image part in the orange rectangle.

As a production tool having the transparent box contain your text, it's really not a practical thing. You would have to postscript code the size of each of your transparent text frames and then overlay the actual FM text frame to fit.

Conclusion: for a special effect, transparency effects can [now] be done in FM. However, from a maintenance and production point of view, it's a total PITA!

P.S. I haven't explored using pre-defined EPS files with fixed-size semi-transparent rectangular areas to match various text-frame sizes. This could make things a bit easier, but it would still be PITA for non-geeks.

Basically, FM is not the tool for this sort of opacity. Whenever I've needed to do it, I've pre-composed as much as necessary in Illustrator.

But there is a way to hack it FM. Frame Art vector objects can be transparent, and can be scaled to arbitrarily small sizes. A grid of small white objects could be created, kept on a Reference Page, copied and placed behind text, and scaled as needed. Relative opacity would be determined by stroke weight. This is a transparent white point cloud.

I just tried it with a grid of tiny white circles. Performance is not great, but it works. A semi-random grid of near point-size single non-overlapping lines at random angles might be ideal (dithered), as it would avoid most of the risk of aliasing and have the least performance impact. If I had an on-going need for such a thing, I'd be tempted to use a spreadsheet to generate the object as MIF, otherwise it's a huge amount of effort.