Hey Blue,
Just got back from flight screening and I had bad cases of asthma as a child (albeit only a few episodes over a year, generally at the change of seasons) so it's not impossible to pass. You have to do what is known as a Mannitol Challenge Test. Effectively you inhale different dosages of powder to dry out your lungs and breath into the spirometry machine after each subsequent inhalation. If you don't drop below the 15% baseline you pass. Some people are a-symptomatic and go through, others may not, I dropped 8% which was within normal regions. If you haven't had asthma symptoms for years you will most likely be fine. I also swam 3 times a week for the year leading up to expand and exercise my lungs so it's something to think about.
As for base visits, while I was down at HMAS Albatross, I just rang up the squadron and after explaining having come back from FSP etc they were going to let me in with a guide then and there until the XO pulled the plug. You have to apply through recruiting, though I know people having gone through so it's worth enquiring over.