I am not quite sure what you are trying to capture here. I take it that you want to use something for very weak/acute patients - but is it at an activity limitaton or impairment level. At an impairment level I don't think there is much else out there.

have you thought about using the Expanded Disability Status Scale? this may be a bit crude for what you want but it probably doesn't have floor effects and would capture the patient's inability to sit.

Have you read this review article? It is getting a bit old but it summarises the available measures well - unless something better has come along since 2002.

Outcome measures to quantify the effects of physical therapy for people with multiple sclerosis | Neurology Report | Find Articles at BNET