I'm pretty sure that you would have to go to the counter, and be assigned a vehicle from the reserved section by the agent. The reservation doesn't have your Emerald Club membership# on it, or the Emerald Club "Contract ID", as they call it. The Contract ID determines what the rate for each res should be.
You could try calling them and asking if they will add your EC# to the reservation. But don't phone the call centre, the agents there don't know ####. Find the number of the location you're renting from, and call them directly. Most of the listings on Google will give you the 1-800 number, which is for the call centre. The rental location will always have a number with that city's local area code. If you search hard enough, you can find most of them...if you can't find it on Google, check the airport authority website for the place you're going to. They are usually busy or too lazy to answer the phone, but I find that if they don't pick up, I usually get through if I try again a little later.
As I'm typing this, I did a search, and found this:
http://www.flyertalk.com/forum/natio...servation.html
https://www.google.ca/search?q=flyer....flyertalk.com
Looks like its a YMMV sort of deal. They probably won't add your EC# to the res in advance...you could still try though.
If you just take a car from the Aisle and drive to the exit, chances are they will make you go back and talk to an agent.
What I would probably do is take my Priceline res and my EC membership card, go to the counter when you first arrive, say you're an EC member with a PL reservation, and ask if you can have your EC# added to the reservation. Once (or rather, if) he does that, then you can ask if you can go and pick your own car.
Chances are, they will tell you to walk to the section of car class you reserved, and pick from there, rather than from the Emerald Aisle. They may let you pick from the Aisle, but it really depends on what location it is, how busy they are, and how the manager is feeling that day.
Keep in mind, regardless, they will probably only let you pick a car thats in the class you reserved. If there is an upgrade sitting in the Aisle, then chances are, it will be off limits.
Nevertheless, it's definitely worth a try. Did you get your EC membership card in the mail yet? If not, then definitely print off one from their website, so you can show that you're a member. Some locations may be accommodating, and others may just treat you as a regular non-EC reservation.
Get ahold of the rental location and see what they say. If you don't get anywhere, ask politely if you can speak to the manager. They might tell you that they can't do it in advance, and to come talk to them when you arrive, or they may just brush you off completely. If they aren't busy or don't have a lot of cars reserved, then the manager might let you take an Aisle upgrade.
And I'm not saying you would do this, but don't act like your EC membership "entitles" you to an upgrade or anything. They get tons of EC members going through, and a lot of them are Executive or Elite tier members. If you're polite, they will be a lot more accommodating, and be more likely to give you an upgrade they haven't been able to move. If they're all booked up though, the best you'll get is the ability to pick anything in your reserved car class.