You can indeed have FCS and Apache both listen on port 80 (we do) - problem is
you may need to have two separate servers. The articles you need to read are:
http://www.macromedia.com/devnet/mx/flashcom/articles/exploring.html Pay
particular attention to the diagrams of the server setup half way down. We have
a setup like that in figure 3. Also useful is:
http://www.macromedia.com/devnet/mx/flashcom/articles/firewalls_proxy.html

I currently have FCS listening on port 1935 and Apache listening on port 80
on my server which is connected via a single static IP.

My DNS table resembles:

foo.com Address 999.888.777.666
test.foo.com Address 999.888.777.666

Apache then serves each website using name based virtual hosting.

Ultimately, what I'd like to do is have FCS work on port 80 in addition to
Apache. I would update my DNS to:

foo.com Address 999.888.777.666
test.foo.com Address 999.888.777.666
fcs.foo.com Address 999.888.777.666

Then using some currently-unknown-to-me technique, have something similar to
Apache's Vhosts to direct the packets to either Apache or FCS,

One idea was have Apache listen on port 905 (or anything other than 80), FCS
listen on port 1935 and have a packet inspector listening on port 80. The
packet inspector would then redirect traffic to either Apache or FCS based
on the 'host' attribute found in the HTTP packet, assuming that such
software exists.

Another hypothesis is create a third virtual host in Apache for FCS which
rather than serving a website, binds itself to port 1935 and acts as a
relay. Again, I am unsure if this is feasible (or indeed do-able)

Please excuse me if I ask the impossible - I am fairly new to all of this so
am not 100% what is realistically possible and what isn't.

Any ideas appreciated.

-Scrawl