imo, sorry I don't have personal experience but I assume we are talking about a client with a degree of paralysis, spacicity, flacidity etc. Ilooked up in Ruth Werner - A Massage Therapists Guide to Pathology - which is a superb bookand always my first place to look.
If I summarize - they are similar, but not the same conditions - Quadriplegia being associated with Spinal Cord Injury, whereas Cerebral Palsy is associated with prenatal, birth or early infancy. They both can respond well to massage - with a strong caveat that there should be nerve sensitivity in the area of massage and good feedback with the therapist to avoid any unintentional damage with massage. Also, especially with SCI you need to be aware of underlying pathologies that may be exacerbated by massage (what caused the SCI - what is the state of healing etc )
As always Ruth Werner is conservative, but I always like to understand the conservative viewpoint before making any treatment decisions. Also typically in either of these cases there will be existing medical care, and it would be good to discuss and get agreement (perhaps written) on massage treatment before starting - again conservative but probably sensible.
Having said that, massage can be hugely beneficial so you need to investigate ways that you can use massage with this client to bring benefits to them. I certainly would not rule out massage just because this sounds like a tricky condition. This sounds like a case where, initially at least, massage should be light and gentle- not deep and strong, perhaps incorporating PMtechniques if you are comfortable with them.
In terms of practicalities - I just think you will need to adapt to the client, and be very careful of your own body mechanics. I would guess that if the client cant move or you cant move the client then its the therapist that will need to change height etc. You might need to sit, kneel (with padding), stand, bend (from the knees) etc.Again no personal experience, but probably good to talk to the care team on that front.
You need to make your own assessment and decisions,and I would also suggest being even more diligent than normal with your treatment notes (just in case).
So, sorry I cant give direct experience, but I hope this helps,
InTouch