I'd like to annotate binary waveforms like you can do for robust and compact by putting text after the colon.

It is not a syntax error to put a colon and a message after specifying the value of binary data, but it doesn't show up. The same syntax actually does something for robust and concise:

@startuml

binary "Binary Serial Data" as D

robust "Robust" as R

concise "Concise" as C

@-5

D is low: idle

R is lo: idle

C is 1:idle

@-1

D is high: start

R is hi: start

C is 0:start

@0

D is low: 1 lsb

R is lo: 1 lsb

C is 1:lsb

@1

D is high: 0

R is hi: 0

C is 0

@6

D is low: 1

R is lo: 1

C is 1

@7

D is high: 0 msb

R is hi: 0 msb

C is 0: msb

@8

D is low: stop

R is lo: stop

C is 1: stop

@0 <-> @8 : Serial data bits for ASCII "A" (Little Endian)

@enduml

I've been using robust as a workaround, but it has limitations (for example, always starting low)