BRU is divided into Concourse A (EU flights except the UK and Ireland) and Concourse B (Intercontinental flights plus the UK and Ireland).
LH and SK have nice new lounges in Terminal A (which was only opened last year). BD have a lounge in Terminal B, which I understand is also used by UA. This lounge is comfortable enough (on the roof of the concourse so OK views), but no showers (BD flies to LHR, a one hour flight at most) or internet access (no BD lounge has net access). LH does have internet access IIRC, but I don't think there are any showers (again, most flights are an hour or less in duration).
The big problem for you accessing LH and SK lounges is getting into Concourse A. This is rather like catching a flight from the International Terminal at SFO and then trying to get into a lounge after security, post 9/11, in the domestic concourse. You will have to clear security and passport control into Concourse B if you have a UA boarding pass. Once through there, you can transfer to Concourse A but it is a LONG walk (with 5 hours to kill this is probably doable). I would imagine you will have to go through immigration between Concourse A and B, as Concourse A flights are all within the common EU travel zone ("Schengen"), and on the way back to Concourse B.
In short, doable but a hassle for some free internet access! The airport may have showers available for public use, which are likely to be OK (BRU is a pretty new, clean place!).
No idea about third party lounges. I know there is one in Concourse B and imagine you can just turno up and pay on entry, but don't know for sure.
Another option would be a day room at the Hilton adjoining the airport terminal - their food is also pretty good. On arrival with UA, you used to get a day room here as the arrivals service, though that has been cut recently.
[This message has been edited by House (edited 03-05-2003).]