Why not just inductively couple to traces, assuming you have microstrip
traces around that you can get to? Then you could use a common MMIC
amplifier to get back up to a decent signal level. I'm thinking here
of something essentially like the microstrip quadrature coupler I
pointed you to, except the coupled line is in a probe, and is quite a
bit shorter than a quarter wave. That just reduces the coupling. You
adjust the coupling by how close you hold the probe to the trace you
want to monitor. (The coupling really is a combination of inductive
and capacitive, I guess.) I often probe with a tiny coil of wire at
the end of a piece of coax, the other end of the coax terminated in a
50-ohm instrument (spectrum analyzer or scope). The coil might be two
or three turns close-wound #32 AWG, a tenth of an inch in diameter.
Just hold it close to traces that carry current and you'll pick up
signal. (It's the cheap & simple version of the coupler idea.)

Cheers,
Tom