Thanks John! It looks like there is also an org-activate-target-links which may narrow the field a bit. Here is what I have so far:

(defadvice org-activate-target-links (after ad-update-target-links ())
  "Activate radio target links as soon as a new target is created."
  (lambda ()
    (interactive)
    (when (and (org-in-regexp org-target-regexp)
	       (not (org-in-regexp org-target-regexp nil t)))
      (org-update-radio-target-regexp))))
(ad-activate 'org-activate-target-links)

Unfortunately, this is not working for me...

Any ideas?

> On 8 Feb 2021, at 13:44, John Kitchin <jkitchin@andrew.cmu.edu> wrote:
>
> I guess the place to do it is with an advice on org-activate-links. Maybe a simple after advice would do.
>
> John
>
> -----------------------------------
> Professor John Kitchin
> Doherty Hall A207F
> Department of Chemical Engineering
> Carnegie Mellon University
> Pittsburgh, PA 15213
> 412-268-7803
> @johnkitchin
> http://kitchingroup.cheme.cmu.edu <http://kitchingroup.cheme.cmu.edu/>
>
>
>
> On Sun, Feb 7, 2021 at 8:48 PM Kyle Meyer <kyle@kyleam.com <mailto:kyle@kyleam.com>> wrote:
> Adam Sneller writes:
>
> > I want to call org-update-radio-target-regexp as soon as org-mode
> > recognises a new <<<radio target>>> has been created (which seems to
> > happen as soon as the third ">" is typed).
> >
> > What hook can I use to get this done?
>
> I'm not spotting any hook that could be used for this.  The <<<target>>>
> fontification happens through a simple 'regular expression => face'
> mapping in org-font-lock-keywords.
>
> You're probably aware of this and it's just not as automatic as you'd
> like, but just in case: pressing `C-c C-c' with point on <<<target>>
> will call org-update-radio-target-regexp.
>