I am totally unfamiliar with such a situation but I imagine clubs such as archery clubs could shed a light on this.
As you have mentioned this will center on training but will probably extend to prevention and body armour and safety equipment as well as correct use of weaponry.
I think that any review of potential issues of negligence could centre on whether there are medical facilities available in situations where it is foreseeable that serious injury could occur.
This could extend to the need to have an air ambulance standing by in the case of life-threatening injury or injuries which could result in permanent disability if not treated within the "golden hour" or other relevant time period.
With any high-risk sporting event where organizers and participants are volunteers [which seems to be two levels below professional and a level below amateur] having a defibrillator available could mean the difference between life or death.
I think you should carry out a full professional health and safety review to give you the answers you need, as opposed to any
ad hoc discussions with other amateur/volunteer groups.
Where risk is foreseeable and injury could be mortal you owe it to yourselves to aspire to and attain best practice standards.
There may also be the issue of special licenses for weapons use, as opposed to discussions with local law enforcement.
This may require you to become affiliated to national or international organizations which could offer you support.
This could also be a requirement of any insurance policy, and insurers often call the shots on other matters.
I doubt that public liability insurance will cover what you are doing - specialist insurance may be needed.
I suggest you search far and wide for information and this link may help -
http://en.wikipedia.org/wiki/Medieval_reenactment
ONQ
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.