Paul 

have been using this reverb patch shown in the image

what it the best way to modulate the floating point numbers boxes over a short period of time?

the aim is to create a reverb unit with a low pass filter that attenuates the high frequencies over a short time period

reverb-modulation-over-time

you should do that in parallel. for example you could split the first reflection and the allpass and use different filters on them.