Get a neck brace or cervical collar and a mouth spreader. Then just secure her in a comfortable position on a bed or recliner where you can position yourself in front of her mouth. Ensure that she's supported from head to mid-back as for instance a pillow under the head alone with a brace on can quickly become painful as the pillow raises the head but the brace holds it straight. The brace will prevent her from turning away and the spreader will stop her closing her mouth.
The neck brace or collar should be an adjustable rigid plastic one for serious injury rather than the soft type for simple sprains, (google will turn up many examples) and they come in either neck height that restricts the head turning, or full neck+head combinations that strap to the chest and forehead to prevent any movement.
The mouth spreader could be as simple as an o-ring gag if it works for your size, or an open mouth gag (plastic insert with or without strap - google is again your friend) or a cheek retractor used for dental procedures like teeth whitening. Another option is a Jennings or Whitehead retractor (medical) but they're not particularly comfortable for either participant.
I have both types of neck brace and both work well. I've been on the receiving end as often as the giving and it's a very powerful feeling when my partner finishes sucking me or pumping me into a glass, and at the moment where I just want to be untied so I can put my trousers on and go to the pub (admit it guys, you know that feeling) she appears in my view (you can't look down with a neck brace on) grinning as she dribbles my cum into my mouth, both knowing there's nothing I can do to prevent it. But then I'm a bit of a perv.