"Newer" squid versions (about 2.6 or so) support using a "nearly the same" log format configuration as apache (see: http://www.squid-cache.org/Versions/v3/3.0/cfgman/logformat.html).
The format required by envision for apache is the common log format(an equivalent logfile entry for squid would be something like:
logformat common %>a %ui %un [%tl] "%rm %ru HTTP/%rv" %Hs %<st %Ss:%Sh
You could then just call them 'apache' hosts. Or use UDS and duplicate the apache xml files and just rename the new device 'squid'.
I haven't tried this out yet, but duplicating apache is what I'll try when I get around to getting our squid boxes into envision. YMMV.