Control of measure_grouping_engraver

[paul]

I am typesetting rhythmically challenging music by B. Martinu. 
A typical feature in Martinu works is a perpetually changing 
time signature. What I'd like to do is rhythmically annotate 
the beat groupings for  tricky parts only, 
for example 4/4 to 7/8 or 2/4 to 5/8. For the 7/8 and 5/8 
bars it's very handy to have measure groupings above the staff. 
However I cannot see how to do this just for selected bars, 
rather than the entire piece. 
Perhaps this is just not possible ? 
The examples on controlling the Measure_grouping_engraver 
in the tutorials are usually in layout 
block and so apply to the entire score. 
It would also be nice to be add grid lines for beat indicators 
just to some bars/measures but not all. 

Is this task possible ?

Regards paul