My first thought would be to ask the LBS's if they know of any hospital-sponsored fundraising rides. Then contact that hospital for info or help.

Perhaps you could contact the medical folks working with your neice?

Several of the hospitals around here sponsor rides. If a hospital near you does rides and has staff dedicated to publicity/educational/fundraising events like that you might be able to get a lot of help.

Do you have any contacts within a Huntington's support group or foundation?