Ok, bear with me here...

How is the front sight base/gas block assembly attached to the barrel on an AR? I was thinking about getting a free float forearm that covers a low profile gas block, and I was wondering if I could just buy a longer gas tube and relocate the low-profile gas block forward from a CAR length to a midlength gas system. I figure it won't matter since the gas block will be inside the FF tube anyway, and I have heard that a mid-length gas system is a little more reliable and 'less harsh' than a CAR type.
I'm not very knowledgeable on the subject of gas systems, so feel free to correct anything I have stated wrongly.

The FSB is either pinned on, or just held on by set screws, or a clamp type....very easily removed.  The only problem with that is the barrel has the gas hole already drilled in it at a specified point, whether it's a 20" or 16" barrel, so you can't move the gas block forward or backward...it won't be over the gas hole in the barrel where it needs to be.    

Now, there are solutions for what you want on a 16" barrel...check out the upper I just built.  I has a low profile gas block, and I used a rifle length tube on it.  I think it looks cool, and the tube completely hides the gas block.  

For a 20 or 24" barrel, I think you're SOL...I don't know of a way to do that, unless there is an ultra long tube for a 20" barrel I haven't seen yet.  

Are you thinking about this for a specific rifle, or just kinda "what if"ing?  

No, just more of a 'what if' thing. I'm trying to familiarize myself with my options, since I haven't built an AR before. I just want to know how much I can do to my rifle before I start building this summer.

Planning is GOOD lol...I need to remember that sometimes!

Yeah, you're sorta stuck with the gas hole where it is, unless you design your own gas system and get the hole drilled in the blank right where you want it.  

Now, I'm not where those "midlength" barrels have the hole drilled...I don't ahve any experience with them.