for many in na methadone is seen as substituting one drug for another and most of us tried substitution and it didn't work.
"the only requirement for membership is a desire to stop using"
you are most definitely welcome in NA meetings... where the conflict usually comes up is some meetings ask that you don't share if you're used in the past 24 hours, to many people methadone is considered using. service positions in na have cleantime requirements and since methadone is considered using, a person on methadone wouldn't meet the requirements for the position.
AND there are some who completely disagree... they think methadone is different because its a prescription....
but the main thing is you are welcome in meetings!
here's a na bulletin written some years back that might help you or at least be interesting for you to read:
http://www.na.org/bulletins/bull29.htm