[Discuss-gnuradio] Capturing maximum complex value

I'm working on a flowgraph that generates an output waveform about 375
kHz wide with very high peak-to-average power -- at least 20 dB. This
is sent into a USRP1 with BasicTX board to generate an output at 29 MHz.
I'd like to show in the flowgraph GUI the peak instantaneous output
level, to help in setting the gain.

It's painfully clear on a spectrum analyzer when the gain is too high.
The attached shows (a) single CW carrier at max output (orange); (b)
output without clipping (yellow); and (c) output with occasional
clipping (blue). These are max-hold plots so the clipping is caught
even though it happens only very infrequently.

Is there a probe or other function that can capture and hold the maximum
instantaneous value of a complex stream? If not, is there a way to
create that capability out of other blocks?

And, am I barking up the wrong tree -- is the distortion likely caused
by the complex output being driven into digital clipping when enough
vectors add up, or is this a function of the analog conversion in the
USRP? i.e., is the problem likely to be on the input or output side of
the DAC?

Thanks,
John