Considering the constraints, I would suggest modified 4x4s. Do you have space to add a small bouldering wall near the systems board? You don't anything big, just large enough to hold several boulder problems, all of which can be overlapping.
The thing about PE training, is that its apparently very specific. So if your goal is to excel at 15deg overhanging crimpy sport routes, the boulder problems for the 4x4 should be 15 deg overhanging and crimpy. For this reason I would avoid using the system board. Of course, if that's the only option, then get some extra holds and put up 4 boulder problems on the system board. Maybe when you're ready for that phase of training, you can remove the system tiles to make more space for boulder problems.
The great thing about 4x4s, is that they're extremely easy to manipulate. If you're training for a particular route, you can build the problems to emulate the route. If you're training for general 8a fitness, I suggest you figure out what your favorite type of sport climb is like, and emulate that. For me, it would be a long, gently overhanging route like The Anti-Phil or The Example. You want the number of hand movements in each set of the 4x4 to match the number of hand movements in your goal route (or route type). So if your ideal route has 60 hand movements, you will want the four boulder problems to add up to that.
Now, 4x4s weren't really designed for that type of route. Usually a route with 60 hand movements will have one or more big rests, so its not a pure power endurance route. 4x4s were designed for routes with no rests, where you are at a full sprint until you're through the difficulties (like running an 800m race). Generally I would guess these routes tend to have less than 40 moves in the business, and it takes 2-3 minutes to climb these sections (and you feel like fainting when you're through).
For longer routes, and general fitness, using a small bouldering wall, I would suggst doing more like a 4x6 (4 sets of 6 back-to-back boulder problems) in order to get the hand movement count up towards 60. The intensity should be less than that of a normal 4x4, and hopefully you won't crash as hard after your peak.
For those that don;t know, here is how a 4x4 works:
Select 4 boulder problems, all near each other, according to the criteria described above. Make sure you have all of the problems dialed before you begin.
With a spotter/time keeper, climb all four boulder problems once. Absolutely no resting in between problems. When you get to the top of a problem, descend safely and quickly and move quickly to the next problem and climb it immediately, and so on until you have done all 4. The time keeper will stop the time when you have finsihed all 4, record the time, and tell you how long you took. Rest for a pre-determined period of time. The goal is to rest for an amount of time equal to how long it took to climb all 4 problems, or around 2.5 minutes. Initially, this should not be possible (if it is, the problems are too easy). I suggest starting with a rest period of 4 minutes, and cutting it down by 30 seconds each week. After the rest period is up, do another set of 4 back-to-back boulder problems. Then rest for the same pre-determined period, and so on. Complete a total of four sets of four boulder problems, for a total of 16 sends. Use the same 4 boulder problems for each set.
If you fail to complete a boulder problem, move on to the next problem, or re-do the problem, depending on how much of the problem you had completed when you fell. Your spotter/timekeeper should help you complete the problems, if this requires taking off weight for certain moves. You will probably need two full rest days after each workout. I suggest only doing one 4x4 per week, but I don't have a ton of experience in this area. They are very intense if done correctly, and you will not feel well at all by the end of the 4th set.
The tricky part is selecting the right problems, especially in a gym where you don;t have routesetting rights. You don't want stopper cruxes, or weird "gym moves" that are unlikely to occur outside. The more the boulder problems simulate your goal route(s), the better. Selecting the proper difficulty is also a challenge. You want them to be hard enough that you barely can't finish the last set. You can also vary the order (hardest problems first, hardest problems last, etc) depending on what your goals are. I suggest that once you start a PE phase, you do the same problems, in the same order, so you can track your progress. The Self Coached Climber has many good tips on 4x4s.