How to type Fraktur with ligatures

Hello,

I have a question concerning typesetting Fraktur texts with Unicode.

Fraktur uses the long s ſ. This is not a problem, one can easily map it onto 
the keyboard.

But the problem occurs when bearing the mind the ligatures.

As written in the “Kleiner Fraktur-Knigge” 
(http://www.e-welt.net/bfds_2003/veroeff/Knigge_digital.pdf page 15), some 
ligatures are mandatory (e.g. ch, ck, tz). Some are recommended, like ff, fi, 
ſt, ſſ and so on.

Sometimes the ligatures are not applied when they occur between two 
independent morphems (?)

For example:

“Katze” and “entziehen”
The first one has a “tz” ligature, but the second one does not have one, 
because it consists of two different parts (“ent” and “ziehen”). I believe, 
this discussion was on this mailing list a week ago or so.

My question is now, how can I control this? I guess the ligatures are made via 
Opentype, so that won’t be a problem, but the problem is how to control that 
at a given position a ligature should not be formed.
Maybe via the ZWNJ? 

Maybe then like this:
“Katze” and “ent[ZWNJ]ziehen”?

I hope you can help me.

Thanks in advance
Gerrit Sangel