[CentOS] This firewall rule will self-destruct

Ok, 

I have a challenge - get two services working over a single port. (!) Sometimes it's a web service, sometimes it's a VNC service. Here's what I had in mind: 

1) Server is normally a webserver listening on port 80, and normally has the following rules active: 
# iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
# iptables -t nat -A POSTROUTING -j MASQUERADE 


2) Server hosts a special PHP script, that when called correctly, sets rules like 

# iptables -I PREROUTING -p tcp -s $REMOTE --dport 80 -j DNAT --to-destination $FWIP:$FWPORT
# iptables -I FORWARD -p tcp -s $REMOTE --dport 80 -j ACCEPT 

The *next* tcp connection from their computer to the webserver will get NAT'd to service $FWIP:$FWPORT. 

But I want this rule cleared out as soon as the connection is made with the latter altered rules. Is there a way to write a rule or set of rules that, when activated, then delete themselves? The "ESTABLISHED,RELATED" rule should keep the connection alive even after the latter rules are killed, right? 

Is this possible? Am I just out in left field? 
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