You can't exactly do that easily.. here's why:
Plex has two versions: the plexpass and public version. Depending on what you have you might need one and not the other. Then you have to deal with changing between the two(what if you pay for 6 months and then stop?), then you have to deal with the fact that the updates may or may not require package updates. Theoretical example: what if plex 0.9.9 required php5-1.05 package but plex 0.9.10 requires php5-1.1? Unfortunately the only way to make a good distinction of that is to have a human look at the errors with their eyeballs and see that the error means you need a newer(or older) version of php5. There is no doubt in my mind that at some point Plex will require something newer than what everyone's template currently has. And then we're going to get a cubic a**load of people asking how to fix their template. And I have no doubt that many people have jacked with their plex installs enough that they will be unable to apply the fix. That's also assuming they are capable of applying the fix. (Keep in mind when that plex updating script came out people asked for youtube videos on how to put it in your jail and run it...). The level of knowledge for your average user is extremely low but their expectations are that they get what "pro's" get.
Literally, the only "good" option is to build the template with static versions and throw it out there for the world. Then updates are from static templates as well(which is why updating Plex has been such a nightmare). That is how we do it now, and it's the only sane way to handle it. If you don't do static templates and you create the jail on-the-fly you run into potential package conflicts, missing dependencies, etc. People already create 50 threads when stuff doesn't work. Imagine if everyone and their mother suddenly couldn't install Plex because of some dependency change when everyone is doing this script update? What if I told you that you have to update an installed package on your plugin and you can't upgrade that package until you remove 2 other conflicting dependencies elsewhere? You might be able to figure it out, but I can guarantee you that 99.9% of plex plugin users wouldn't be able to fix it and they *will* expect a step-by-step guide, and they *will* expect it immediately, and they *will* expect it to be provided for free.
My personal take on this whole thing: provide zero support for what you do inside the jails and with the plugins on the forum. We support the jail framework and that's it. What you do inside of it is totally and completely your own chosing. Get rid of it all.. you got a question, don't ask it here. To be honest, what goes on inside the jail(or plugin) is nothing more than freebsd stuff. This forum is not prepared to handle nor are there enough experienced users to handle the workload of everyone asking how to do everything in freebsd. That's exactly what we get right now, and it's why I just close my browser tab when I see them. Do I know the answer? Quite often I do. But I'm not about to start teaching 4/5 new users how to use FreeBSD. That's just not possible when there's only me and only 24 hours in a day. Not to mention 99.9% of those 4/5 users wouldn't even consider paying someone to do it, and I'm not about to devote my life to other people's jails/plugins for free. I do like having a roof over myhead.
Anyway, when crap goes bad in the jail people come here to post. Do you think people that can't run Windows in a VM and go to the VMWare forums complaining get assistance? Hell no they don't. The thread is deleted. Now when you post that Windows is running, you've narrowed down a networking problem to something that revolves around the VMWare virtualization framework, *then* you get their attention. Until you actually prove that though, they don't want to hear from you. They don't care what you run in the VM. They only care about the framework(and rightfully so). VMWare isn't about to support every OS on the planet just because you can run it in a VM. Yet we're somehow of the idea that we should support everything that can run on FreeBSD because it can run in our virtualization framework.
And for the record, when people complain that some program doesn't run in the virtualbox hack do you know what I do? I delete it.