Yes you can exceed the 13 characters but only if you “hack” the format.
The following works because it uses special coding to send 17 characters in a 13 character space…
CQ DX VK3AFW QF22.
As the format allows for a 4 character prefix, which is handy as
CQ SOTA/VK3AFW QF22 also works
That’s 19 characters.
But there may be regulatory issues with such a prefix. I don’t expect a blue ticket if I use it on VK but maybe elsewhere it would not be accepted. It does show there is scope for “hacking”.
To avoid being in trouble I suggest setting up your actual prefix if appropriate and the summit grid square. Select standard messages.
Then add some extra things to the message 5 “bank” to be selected as you please. I will have the summit reference and 73 which just takes 13 characters plus the alternative CQ call as above. That way I can run in standard format but mix in the extra info and put out a different CQ call.
The receiving software figures out which country you are in (assuming you used the standard message format) and adds that to your CQ call so that’s a reason to maybe alternate the standard message 6 and the hacked CQ SOTA/ version in the message 5 bank.
So you might see CQ VK2/VK3AFW QF44 AUSTRALIA
which is a whole lot more than 13 characters.
Once QSO starts you need to bring up the modified 73 message in message 5 to avoid upsetting the logging yet still give your summit. You can add any other summit details to the log for S2S.
The bottom line is you need to have your call (and prefix) and grid locator in some CQ calls for spotting on FSK Reporter and the Summit Reference Number for the chasers in at least one message.
It’s not all totally automated!
Unfortunately 1800 hrs is a bit early for me but VK is a possibility on 4o m at that time.