The good news is that it sounds like you already have the hard part of any recording
system completed. I'm assuming mic's are hung, your board is able to handle an output and you're ready to record. I do have to say that it is a better idea to isolate a separate mix of your sound for recording if you're ever planning on using wireless mic's. The levels for a wireless show (when properly mixed) are constantly being adjusted depending on who's singing at who when and how. If you're planning on doing straight recordings using just installed mic's then using a tape out is your best bet. Otherwise I would opt for your original aux idea.
Using cassettes to record isn't an unheard of idea, and before the digital era was quite common. (I remember when BETA used to have the superior audio format and people would record sound effects using their videocameras.) But you want to stay
current with technology to the best of your ability. Especially if people will be learning sound on this
system, you don't want to be teaching something that has floated so far from industry standard as to recieve ridicule (which is where cassettes lie at this
point.) Plus they are something you have to
purchase a consumable material for, and end up costing money just to operate. For the same financial cost I would highly consider a baseline
Mac mini. It comes with a free program called GarageBand which may not be in the realm of PRO Audio, but posesses all the basic principles and functions of any high performance Digital Audio Workstation (DAW) such as ProTools or Logic (software used in the professional sector for recording.) It will also allow you to upgrade if you ever find the desire to do so to something more professional grade. The
line you really sound like you don't want to cross is spending money though, which is understandable. The world of sound is the last to have reached independence in the world of theater, but has by no means been undeveloped. In concert venues, sound systems easily enter into the hundreds of thousand of dollars range, and all the exact same technology is now being discovered by theater venues. What Theater may see as a "unreasonable" price is astronomically cheaper then what industry standard has been set as for the last few decades, and with the advent of computers, the 'home recording studio' has become a very recent reality.
(Sorry for the history rant, I just completed a seminar for theatrical sound
system design where many of us shared industry stories dating back about two decades
)
Long story short, yes, cassettes are acceptable and they will get the job done, there are many reasons this will become a hassle for you in the long run.
If there is enough interest in providing recording, I would gather information on some equipment and propose it to the powers that be for an annual budget consideration. Rushing into a quick solution may only end up spending money that could be set aside for something more useful and constructive. Digital audio IS where audio has found it's future. Analog is nostalgic at best.
Hope this helps!
-Jon Bremner
Resident Designer,
"Generic Theater," Norfolk, VA