@ Ron VK3AFW
1) CQ format
I tend to think both the CQ W6PNG SOTA and CQ SOTA/VK3AFW QF22 should be avoided independent of any regulatory compliance. To me, both messages are a little confusing for the uninitiated. With what I tried potential chasers may wonder if SOTA is a grid or just wonder why a grid isn't including. With yours and to your point it is bastardizing a legitimate format such as J6/W6PNG or MM/W6PNG.
2) Including a grid in CQ line
WSJT-X will do this in the auto generated messages but its not essential for a QSO. I agree PSKReporter et al will key of this but that in essence is a nicety for us, the activator, in knowing where we are being heard. Last week I was thrilled that 7 watts FT8 from Wyoming was heard in Tenerife of the West African coast. I'm not sure it will drive chasers to chase me but its early days so I could well be wrong. During the eight FT8 activations I did last week in CO/WY the vast majority of chasers have come because I spotted myself to SOTAWATCH (I know the callsigns are SOTA chasers) and I presume the remainder from staring at the left hand decode page on WSJT-X.
3) 13 character limit
I certainly have seen messages that are longer than 13 as in your example CQ DX VK3AFW QF22 (17 to be precise)
I still need to do some homework here as;
3.1) If you hover your mouse over message 5 in the lower right hand side of the WSTJ-X main page it displays a pop up box that reads "Enter a free text message (maximum 13 characters).....". I guess the CQ DX.... is defined via the TX Macros in the configuration page and then made available in message 5 as drop downs.
3.2 One place that I seemed to momentarily mess up on was in my free message exchange (set via a macros) that was of the format "PEAK W7Y/SW006" which seemed to yield "PEAK W7Y/SW00" on the air. I lost the trailing 6 for a total of 13 characters. Guy N7UN suggested I shouldn't be using / anywhere outside of a real call sign.
4) Modified 73 - I agree this is possibly the best way to exchange the summit reference. The biggest draw back to dropping their call sign in the 73 is it messes up the auto message sequence and isn't highlighted in red (or whatever color you have configured) possibly causing confusion
Bottom-line for me is to keep experimenting with both FT8 and WSJT-X as an activator and build awareness while gravitating toward a good workable message exchange.
Ron, thanks and maybe I can chase you across the 7,000 miles of so of water to the immediate left of my QTH.