JayB, I agree. It is nonsense if the only bit of wall on Station Approach (as I read it) that the council "prefers" is the short new stretch behind the Tardis as I can't imagine any artist wanting to use that space which is mostly hidden anyway. Plus the council has done absolutely nothing to improve the Tardis itself despite howls of protest and many pleas to do so. It is one thing for the council to refuse to improve its own eyesore, but hypocritical to prevent a potential crowdfunding effort to improve something privately owned.
So, if it is indeed the case that the council's lack of clarity is holding things up, maybe an organised placard waving group could be arranged and notify the local press just before the next Sydenham Assembly meeting on 13th (and notify the council in advance accordingly). Someone organised one or more such demonstrations to save The Greyhound as I recall.
JRW, I lost track of which site you were referring to at points in your post, but two demos would make it even more worthwhile for the press to attend
Hopefully the council would then be obliged to stop being vague and state clearly and specifically at the Assembly Meeting or otherwise which rules they were applying, if any. If no rules apply, also to give an assurance that artwork would not be removed (or owners instructed to remove any) if on private property.
Of course, if it's not the council's lack of clarity holding things up, then maybe something else is...