Using a custom search path with Search API

Problem/Motivation

I am trying Search 404 again with Search API → but and I am hitting the same problem I did last time I tried and gave up :-(.

When I go enter the following...

/search?search_api_fulltext=@keys

(My particularly simple Search API path in play there.)

...into the 'Custom search path' input, I get the message...

Custom search path should not contain special characters other than "/?=_".

It seems to me that the ability to include the '@keys' in a custom path is crucial if we are using Search API since the search keys need to appended to the standard Search API URL yet, it does not accept it?

I recall in the past that here may have been a fix for this in the form of patch somewhere in here → but there's so much going on in that thread that I cannot fathom it out.

Am I missing something terribly obvious here or am I correct in saying that using Search 404 with Search API is indeed blocked?

Any help here gratefully received - perhaps there is a patch somehwere that'll fix this and if there is, then surely it should be in the next release without any hesitation?