Hi Froggy,
I'm quite sure that the people who have asked you to volunteer would be quite willing to brief you and give you advice about the kinds of illness you'll be faced with, as well as answer any questions you have and provide you with the support you need to do your reflexology treatments. Have you spoken with them and discussed your concerns? There is so much information out there, readily available to the public, speak to MIND, get a feel for the type of illness you'll be working with and then research those particular types.
I'm also quite sure that you will not be left in any situations that compromise your physical wellbeing, however, ultimately you are still responsible for your own safety so I would check this all out with them thoroughly before.
I think you'll find the experience hugely rewarding and will gain much from the people you meet and the learning along the way. It certainly will be challenging at times, but hey, life is about challenges and opening our minds and hearts to places and people we don't ordinarily come into contact with everyday!
I've worked with people with mental illness and can assure you, their feet are no different or no more 'unclean' than anyone else's.
I've met some of the kindest, most gentle souls with an array of mental illness; from schizophrenia to very mild depression and they are still hearing, breathing, feeling human beings, very much needing assistance with their lives as a result of their illness. Very often they feel isolated, lonely and rejected and the loving comfort that comes from the touch of reflexology or massage is so greatly beneficial to them.
Alot of people with mental illness are in a great deal of distress and to suffer from delibetating depression or the apathy associated with certain schizophrenia's is far more distressing to them than can be imagined by those of us who take getting out of bed every morning for granted. Try to remember that these are people in need of support and care and you're in a priveleged position to provide a part of this, it is an honour to assist and support.
Try not to worry too much Froggy, ask MIND to support this process with you and enjoy the learning and the experience of making a difference in people's lives. You'll get so much from this experience and touch many lives.
Good luck,
x