I have now moved away from all things PADI, but have a credit for a couple more classes. I'm thinking now, they couldn't hurt, and at least it means I get some boat dives in without paying (more) for them. . . . so I'm hoping it's a little more informative than the typical padi class.
While I think you should probably dive 'elsewhere', given the comments above, the PADI agency has no rules that would prohibit you completing the class in a SM configuration.
Will a PADI instructor permit me to dive the class in this configuration??
Possibly. Possibly not. But the answer has absolutely NOTHING to do with the agency affiliation of the instructor. You could substitute 'NAUI', 'NASE', 'SEI', 'SSI', 'SDI', etc for 'PADI' and ask the same question. And the answer would be the same. It depends on the experience of the instructor and their comfort with the configuration. I have dealt with non-PADI agency instructors who would not let a student wear a backplate in a recreational diving class - because their view was that a BP is strictly 'technical diving equipment'.
The instructor is away until next week, and I need to decide if I'm taking the class or committing to dive elsewhere, so I'm hoping someone can tell me if PADI has rules about this sort of thing.
So, you are saying there is absolutely NO WAY to communicate with the instructor prior to the class? Seems a little unusual in the 21st century, but I guess it is possible. That in itself would concern me, about the instructor, individually. I don't make myself unavailable to enrolled students in a course the week before a class starts - in fact, I am usually initiating communication with them (and getting frustrated when they don't respond until the first night of the class, when they give me some lame excuse about how busy they are at work).
The PADI agency has 'standards' regarding required equipment, as would any competent agency. But, the standards are fairly inclusive, as they should be. There are no 'rules about this sort of thing'. Rather, the question is whether the specific instructor is comfortable with you diving in his / her class in a SM configuration. If the instructor has NO experience with SM, they might ask that you use a configuration with which they are familiar / comfortable. That is perfectly reasonable and also independent of agency. I teach SM (PADI). I would be happy to have anyone dive, in a course that I am teaching, in a SM configuration, IF they are competent to do so. Frankly, your card (PADI / NAUI / SEI / PSAI / whatever) doesn't tell me that you are competent. Before EVERY con-ed course, I do a quick evaluation, in / under water, of every student, whether they are diving backmount / sidemount, whether they are using a jacket / back inflate BCD, whether they are diving a double hose / vintage system, etc. As a related example, I am OK with divers showing up with a FFM, because I am familiar with FFMs, and certified in their use, even though I don't teach FFM myself. I have used a rebreather, but I am by no means facile with one. If you show up for a class I am teaching in a rebreather, I might have concerns - about me as much as about you. That has nothing to do with agency standards, rather it has to do with my ability - as the responsible instructor - to be functional in an emergency.
Don't just show up in a SM rig, unless you are simply trying to prove a point / make a statement. Communicate in advance, it will make life easier for everyone. Chances are it will be a non-issue.