The User Showcase is an area where users of PG Music products (e.g. Band-in-a-Box or RealBand) can post links to their original song compositions, for others to listen to.
The songs must be originals, no copyrighted or "cover" songs. You must have all of the rights to the songs.
This is PG's rules. The exception is if you have purchased the license to record and publish the song publicly .... in which case you need to include the license number in the original post so it's legal.
Folks use BB to replicate covers all the time.... but you're not supposed to post them on line without permission from the copyright owner.
Regarding PG forums...their forums, their rules! But for other purposes don't let the idea of procuring a license put you off. Typically not a big deal nor a big expense! In the US you do not need permission to record a cover of a song that has been commercially released. But you do need a license and be set up to pay royalties on all sales.
Make a video and post it on Youtube and don't worry about it. One of two things will happen. The artist/record company will take it down - this rarely happens anymore. Most often they'll put an ad on your video and an ad on your video page where the listener/watcher can buy the original and they get the money. You cannot monetize a cover video because it's not your copyright.
A few years ago this was very different but today most labels are actively encouraging covers on Youtube because of the exposure and extra money they make from them. And technically you are supposed to have a very expensive sync license to do covers on Youtube but honestly no one seems to care anymore and it's rarely enforced. As a matter of fact from my research if you are a popular cover artist who gets the views labels will come courting you and it's even possible to make a few dollars for yourself with a deal to split some of the money. But it's very hard to achieve that status.
One more thing, if you're not into Youtube and want to release your covers for sale and post the songs on sites like this one and your own website and get it on iTunes etc., you can get the license for no money up front. Check out www.loudr.com DISCLAIMER: I have not used this service yet so due diligence on your part and understand what the terms are before you agree to anything.
BMI AND ASCAP collect the royalities and pay the copywrite owner. There is a standard collection fee, usually 12% and 80% to the original copywrite owner. This means there may be 8% available for you. You will have to register with one of them when you publish the song. It will be assigned a barcode upc. Don't waste your time on cover songs for a profit. BMI /ASCAP will also be the one you will get a letter from the courts advising you to appear if you publish a song for sale. If your just doing cover songs for fun an not for profit, you'll be ok.