fieldmarking question?

[Thomson, Martyn EDUC:EX]

I have a screen that uses the fieldmark action - 

actionmenu label "Modify" action fieldmark menukey "M" 

It enables the client to arrow from label to label and then hit ENTER to
access the field they wish to change.
If the field statement has the "nochange" option, they get the message
"Can't change the specified field(s)".
My question is - is there any way I can make access to a field conditional
in this scenario (i.e "nochange if..."
aka "entry if")?
I can't see a field option or anywhere I could add code to intercept this.
There's no generated modify procedure 
I could tweak. I could put a test in the input procedure, but I want to make
the field appear unavailable rather 
than generate an error message after the event.
Is this at all possible?? is there an alternative approach?
The only alternative I see and want to avoid is to move everything but the
key input field to a subscreen and 
have 2 lookalike versions of the subscreen - one for each result of the
conditional test.
Martyn Thomson
7.10.G1 on OpenVMS AXP v7.2-1, Oracle Rdb V7.0-5.