The only easily available web server in RouterOS that you can customize is the Hotspot one. How to use it to serve arbitrary pages is documented in the forums, such as here:
http://forum.mikrotik.com/viewtopic.php?f=1&t=53691
The basic idea is to have an interface (could be a loopback - an empty bridge with no ports in it but an IP address assigned) and run a Hotspot on that interface. You can then put arbitrary HTML into that Hotspot directory and load it from anywhere.
The rest of it would look a lot like the payment reminders documented here:
http://wiki.mikrotik.com/wiki/Payment_Reminders
So to sum it up: first create a loopback interface and run a Hotspot on it.
/interface bridge
add name=loopback0
/ip address
add address=172.31.255.255/32 interface=loopback0
/ip hotspot
add interface=loopback0 name=loopback-hotspot profile=default
Then upload any HTML to the directory created for this purpose (you'll see it in the file system, the folder name is taken from the 'html-directory' parameter of the profile found under "/ip hotspot profile", which is referenced by the Hotspot instance in "/ip hotspot". Let's assume you uploaded troubleshooting.html, and the directory is called 'hotspot'.
Then configure the proxy to redirect to that resource, and redirect all HTTP traffic coming into ether1 to the proxy.
/ip proxy set port=8081
/ip proxy access
add action=allow local-port=8081 disabled=no dst-address=172.31.255.255
add action=deny local-port=8081 disabled=no redirect-to="http://172.31.255.255/hotspot/troubleshooting.html"
/ip firewall nat
add chain=dstnat in-interface=ether1 protocol=tcp dst-port=80,81,88,8080,8888 action=redirect to-ports=8081 comment="redirect if radio is down"
Then adjust your scripts to enable the NAT rule when the radio is down:
/ip firewall nat { enable [find comment="redirect if radio is down"] };
And disable it again when the radio is up:
/ip firewall nat { disable [find comment="redirect if radio is down"] };
That should do it. Completely untested, though.