I have not tried this with the proxy setting, just so you know...
I FTP uploaded a text file called test.html to the hotspot directory for this test, but you should be able to upload it anywhere.
I use a hotspot with IP 192.168.0.1 on wlan1 for this test.
displays the test page correctly.
Considering that works, I would try this:
/ip proxy access set X redirect-to=hotspot/test.html
where X is the line number in '/ip proxy access print', and set 'redirect-to' to the correct directory/filename
If that fails, then try the IP of the interface to the MT box.
/ip proxy access set X redirect-to=192.168.0.1/test.html
UPDATE: Just tried it with the proxy. It must be the IP or URL, not the directory/filename. And it will not work without the hotspot webserver. I disabled the hotspot, and it will no longer redirect. I suppose without the hotspot, it must be on another box with a webserver.