I'm assuming you already know about bikes being strictly banned on the trails at Griffith Park... same thing applies here. You absolutely do not want to route people on bikes onto that stretch of access road beyond where the newest section of bike path ends; drama will ensue:

https://www.latimes.com/socal/burbank-leader/tn-gnp-me-mariposabridge-20160130-story.html

Last year there were signs of renewed interest in doing *something* to continue the bikeway, but the ideas presented seemed pretty unlikely to work out IMHO:

https://ladotlivablestreets.org/projects/LA-Riverway-Phase-IV

I'd love to see something happen with that, but I'm not holding my breath. Meanwhile, that section should be access tagged as horses and pedestrians only. Seriously, people have been traumatized by those equestrians.