Hi @certley ,
and welcome to the ROOT forum!
Allowing expressions in place of column names could be done; on one hand it would save typing for users, on the other it would encourage some not-so-nice usage patterns and complicate internals. See also my reply here: Create Histogram of composite variable in RDataframe - #2 by eguiraud .
I have been thinking about it and a possible middle ground would be:
df.Histo1D(ROOT.RDF.ColExpr('px[0]'))
but I don’t know if it’s worth it 
Cheers,
Enrico