nesting tags--using an MT tag as an attribute for another tag

Greetings,

May be kind of a silly MT n00b question, but I've been searching for a couple days now and haven't found my answer, so it's time to ask directly.

I'm working on a plugin to get comments for an entry, and trying to call it from a template like so:

>mt:ListComments entryID=">$mt:EntryID$<" <

In my tag handler, I've got

my ($context,$args) = @_;
my $entry_id = $args->{entryID}

From what I've been reading about templates and template tags, it seem like this should work, but it's not happening. My handler gets called OK, but in MTOS, my $entryID variable contains ">mt:EntryID<", in MT Enterprise, it's blank. I've tried using >mt:Var< to set a variable, but using that inside my plugin tag doesn't work either.

The basic question, then: is it possible to use an MT tag inside another MT tag this way? If not, how can I get my EntryID value to pass as an attribute to my plugin tag?

thanks,
Bill