I've gotten several emails regarding this post, so I figured I'd just
post my current work-around.

The general idea of the work-around is to set up proxy subscriptions.
For every dynamic topic that you want to have, you set up a proxy. To
do this, I had to add a couple of extra calls to my dll.

int GenerateProxyId(unique id) - This function generates a proxy id and
binds it to some unique info. For my unique info, I use the workbook
name + cell name of the thing that is calling GenerateProxyId. You
need to do this so that when you open multiple workbooks in the same
excel instance you don't get id conflicts between workbooks.

DateTime AttachData(proxyId, data) - This is a call that I use to
attach dynamic data to proxy ids. I return a DateTime because it is
kind of useful to know when the last time your dynamic topic changed.
I keep a hash of proxyId-last change time so that if AttachData gets
called and it is the same data or there is an error condition or
something, I can just return the last good date.

I also added a special branch through my code to look for a special
topic "proxy". So when I get a call like
=RTD("My.RTD",,"proxy",3,"#bar") In this case, I would see that it was
a special "proxy" topic using the proxyId 3. I would look up the proxy
data 3 and find something like "dynamic-foo" and then attach it to the
static part of the subscription "#bar" and get something like
("dynamic-foo", "bar"). By having the ability to combine static data
with dynamic data, you can save some proxy/attachData calls. For
instance, if you wanted to have 2 subscriptions (stock, "BID") and
(stock, "ASK) where stock was a dynamic value, you would just have the
following:

=GenerateProxyId("my workbook.xls", "A5") -- 3
=AttachData( 3, stock) -- 12:03:05
=RTD("My.RTD",,"proxy",3, "#BID")
=RTD("My.RTD",,"proxy",3, "#ASK")

* I use the # symbol to indicate weather the proxied data should go
#before or after# the static data.

That's about it as far as the solution goes. It's a very painful way
to do something that seems like it should be there out of the box. The
code gets pretty ugly too. With the static data + dynamic data
combining, you have to do several one to many mappings and reverse
mappings.