They could always call the midwife...give this 'collaborative' stuff an actual try before deciding they can't collaborate with an IM.

The IM would know the dosage. Which they would cover in the letter. It might be easier for the doctor to see the IM, but I'm sure the IM has other things to do, not to mention the unnecessary cost to the mother for an extra consult? Perhaps Maya is right, and it really is a power thing - if my GP wrote a letter, my specialist wouldn't be saying 'hmm not sure I can verify the authenticity of this letter, can you bring him in to chat with me?'

Certainly easier access to the necessary drugs for IM's would make this whole thing a lot simpler and less discriminatory. As Trill says, planned freebirths are a whole different ball game, I'd be advocating a freebirther transfer with good advocacy/support in place if there were complications.
But if there were concerns with a doctors identity, they can always check their medicare provider number to verify, if there was such a system for midwives that made it easier to verify a letter from a professional IM if the doctor had any doubts. Yes they could check with the nurses board if they are registered as a midvife, but it wont tell them if they are a practicing independent one. I am not a midwife but a registered nurse and but I can access drug dosing information and use the right terminology to make a letter look legtimate, forge a midwife colleagues name and find out her regsitration number and provide that as well if I wanted. It is not hard.
I have seen some GP's referral letters and I would want to see them to for some of the stupid and unhelpful information they provide (as in please see patient, she is complaining she is unwell). As well a specialist should examine you and come to thier own conclusions and not just rely on a GP's letter.