I thought that UN pilots were peace keepers on secondment from the various armed forces.
The other most common activity is disaster relief and aid agencies. Again, the largest supplier of aircraft is the miliatry, eg the helicopter (Sea King??) that crashed in Aceh whilst assisting with the tsunami relief was from the Australian Navy. If eye sight is a problem, I think these options are out, pity though, because it sounds exactly what you are looking for.
An often missed option is working with the various church bodies. One that I know of is Mission Aviation Fellowship (M.A.F.) MAF USA has helicopters, but MAF Oz does not. An associated company is Jungle Aviation and radio Service (JAARS).
http://www.maf.org/ is the US link.
http://www.jaars.org/aviation04/avia_home.shtml is the JAARS aviation link. Most of their work is done under the banner of assisting missionaries and bible translators, but they also do jungle EMS stuff, simply because they are there. JAARS run a longranger. I don't know what MAF are using. MAF looks for 1000 hours for the plane drivers, I don't know what the hours are for helicopter, other than I didn't have them last time I looked!
If you do a search on Google you may find something.
These agencies use CPL holders only, no private pilots, so you will still have to pass the class one med.
Using this site is a great way to get more info.
(Edited to add web links)