I really like the idea of enduro racing but I cant currently afford an enduro specific bike. Is it acceptable to enter an enduro event on an aggressive XC/trail hardtail mountain bike? Are there any rules against it or would it not be recommended due to the terrain?
Yes, no*, yes.
Enduro racing is a hybrid sport, but the timed aspects of it are almost all downhill. As such, you're going to want a bike that can handle that the best, which would most likely be a full suspension frame. There aren't any specific rules (yet) against hard tails, although a few events may dictate no hardtails for their race specifically.
One thing that I would consider essential is an adjustable seatpost with a remote, to lower your seat for downhill sections. This would be both for comfort and safety, as well as the speed of not having to climb off, lower/raise seat, get back on and go.
I don't know if you currently have a bike, or are shopping, but the newer rear shocks are designed to help with pedaling, especially on uphill sections without needing a lockout, so even if a majority of your pedaling is on easy trails, it won't hurt to have a full suspension for everyday use, and then be able to use it for enduro as well.
Added: So, yes, you can probably use a hardtail for enduro, however it may not be the best fit, and you run an increased risk of injury and/or frame damage depending on the course, technicality and skill of the rider.
Yes, unless they say no.
The "Spirit of Enduro" is to include all riders, of all abilities, on all bikes. Since there are guys who can ride crazy fast on a hardtail down a downhill trail, you can certainly ride yours in an Enduro race. However, you might be putting yourself at a disadvantage compared to the pro's or if you are looking to be competitive.
That said, if you take a carbon-fiber XC race bike to an enduro race, crash, and break the frame, that's your own fault. There is such a thing as having the right tool for the job. Taking an AM hardtail with a more raked-out head angle and med/long travel fork would be the best thing.
There's also the trail itself and this can vary widly. Within one race I rode trails that were fairly tame, but fast and flowy, to trails that were super steep and loose with a lot square edge rocks and whatnot. An all around trail bike will be a happy medium here. Plus, look at how much climbing you will be doing. A 2 hour pedal to the race start on a mushy 150mm FR bike isn't that fun (trust me, I did it), where as on a hardtail you'll just think of it as a nice warmup!