Yes.

Any of the various remote empeg apps that work with Hijack can do this, including charcoalgrey's web interface (and derivatives), Tony's Empface (I think -- never tried it here, cuz it requires a $200 operating system), etc..

Or even a simple web browser or command-line tool like wget.

http://your.empeg.ipaddr?button=volup
http://your.empeg.ipaddr?button=voldown

Cheers