TextMate can use either the file extension or the first line of a file to divine what filetype it is.
For my diff bundle (I think it's the original), this is:
firstLineMatch = '(?x)^
(===\ modified\ file
|==== \s* // .+ \s - \s .+ \s+ ====
|\d+(,\d+)* (a|d|c) \d+(,\d+)* $
I don't have mercurial, but it appears as though its diff's first line may look something like
diff -r 2278160e78d4 hello.c. As such, you can add another line to that big OR switch:
Note that the final character in the line is a space.
As another alternative, it appears as though you may be able to define your alias as
alias hgd='hg diff --git|mate' to use the same format as git, which is already recognized by TextMate.