Another late reply....
Last time I went to mass (a very long time ago), the whole thing lasted over an hour. Maybe this was mass with extra bits; not being a catholic, I wouldn't know (I was there as a clarinet-playing atheist). I have heard it said that a properly-done mass should last at least 20 minutes, otherwise you are rushing it, and gabbling your way through a religious service is not respectful, let alone worshipful. 20 minutes is also about the limit of most members' of the public's attention span for something they don't particularly understand (or in some cases, relate to) - especially if it's in Latin. Rather than a full mass, we might be better off with something shorter.
Re-enactment in general, however, could do with a bit more religion sloshing around. It was so prevelant at the time (whatever time you are doing, right up to about 1960) that not having it leaves a huge hole in our portrayal of society.
Regarding people finding a respectfully-done portrayal of a religious service offensive, I would be interested to know whether these people also find portrayals of religious services in films offensive? If not, why not? To some extent, a bunch of re-enactors in a field is no different to a bunch of actors on a film-set. It's all pretend.
Another angle to be considered is, do the outraged feelings of people who are offended outweigh the feelings of those people who would like to see such things? After all, the outraged person can simply go away and not watch... the person who would like to see what a mediaeval catholic mass was like (as far as we can tell) doesn't have a choice if the portrayal is not done. The old risk/benefit ratio thing rears its head again.
To some extent, whatever you do, you're going to offend someone. Religion is one thing. Meat-eating is another - vegetarian members of the public have been known to be offended by groups having meat openly on display. Anything military is something else again. People have the capacity to be offended by almost anything. So you have to decide, how many people are likely to be offended, and how badly? And do you, or do you not, think they have a point?
If you are not inciting religious hatred, and you are not holding someone's beliefs up to ridicule - you are, in short, doing your level best to potray a religious practice in a balanced and accurate way - then the decision is up to you. Some groups will choose to go ahead, and some groups will choose not to.
Of course, the obvious way out of all this is to have a recruitment drive targeted at catholic priests. Get a real priest to do a real mass: problem solved; no offended catholics.
...Of course, this will then offend the atheists, who would be happy to attend a pretend mass but would be outraged to be expected to go to a real one.
This is why religion is not discussed at dinner parties: it's impossible to please everyone.