Eraserhead nailed it with the Barry Stark video. Absolutely, positively legal, nothing dodgy about it at all. This is a case where unfortunately, I find myself in disagreement with Steve Davis even. Not a grey area of the rules at all.
The Rules, as currently written, absolutely allow this sort of thing and personally, I see no reason at all why the Rules should be changed to disallow it. Something that one should always keep in mind whenever one is trying to calculate the legality of any sort of situation in regards to the Rules of Snooker: the Rules are purposefully written as a minimalist document. As such, this means that if something is meant to NOT be allowed in the Game, then it is specifically written as not being allowed in the Game. If there is nothing written, then quite simply, it is allowed. A perfect example of this is Section 1., Equipment, Rule 3., Cue:
"A cue shall be not less than 3ft (914mm) in length and shall show no substantial
departure from the traditional and generally accepted shape and form."
This rule was written as it is because sometime back in like the 1920's or '30's, somebody was in a situation that he wanted to cue low and parallel to the table, but the White was a half-foot away from the cushion. The player pulled out what was essentially a miniature replica of a cue stick the length of a biro with an actual tip on it and played the shot. This was perfectly legal but seemed quite odd at the time. Thus, a new Rule was born, that which is written above.
I had not seen that particular Barry Stark video before though I have found his information to be very accurate and useful. Thank you for the post Eraserhead. A few things specifically about the video...
Right about the 3:00 mark, Barry mentions that the legality of this technique is under debate. I personally am on the side that this should remain legal, no reason to change. The reason that I hold this opinion is because of what Barry says then at about the 3:27 mark...."...providing this piece of equipment is recognized by the EASB as a legitimate piece of equipment." (The WPBSA Rule Book states, "...All extensions, adaptors and other devices to aid cueing must be of a design approved by the WPBSA Ltd.") THAT is the important bit in my opinion. So if you were to invent a device that would steady your bridge hand for this type of shot...say something that clamps to your forearm and lowers down a tripod onto the table surface to stabilize the whole set up...well then, THAT would clearly be illegal because your clever invention is not recognized by the Rules Authority.
In fact, if an actual movement is made to outlaw the use of Spider this way, then I would add that it ought also be illegal to perform the stroke that Barry does at about the 2:00 minute mark in the video...that is, using both the Spider and the standard rest for a single stroke. That is an old trick that I have known for decades and certainly, that should not be illegal in my opinion, though one ought to think it should be illegal if Wilson's stroke ought to be illegal.
In fact, there was a parallel circumstance some years back as I recall. Apparently, many believe that Ronnie O'Sullivan should have been penalized as he seemed to use a cube of chalk to somehow check some relationship on the table or something. I did not see the incident and I have never found a video of it so I don't know exactly what happened. (If anyone has a link, I would appreciate it.) At the time, the incident passed without penalty apparently to some debate among fans. (I have no opinion as I don't know exactly what happened.) So this is a parallel case in that a cube of chalk is obviously "approved equipment" but the Rules, specifically Section 3., Rule 10., Penalties, state:
"... Penalties are:
(d) seven points if the striker:
(ii) uses any object to measure gaps or distance;..."
So if Kyren Wilson's tactic is going to be made to be illegal, then in a similar way, the Rules will need to be amended to say something along the lines that approved equipment may only be used for intended purposes and no other. Personally, I think this area of the Rules is just fine as written and Kyren Wilson's shot was and should remain perfectly legal.