Brussels Airlines lounge has shower facilities:

Brussels - Brussels Airlines Business Lounge

There are two lounges at the airport in Brussels, one in Terminal A (just in front of gate A42) and one in Terminal B (just in front of gate B01). The lounges are open from 5am until 10pm.
Services:

Check-in facilities
Upgrade with air miles
Information about Privilege
Information about your current balance of air miles and how to use them
Assistance if there are any problems
Facilities:

Free internet access
Free dial-up access for laptops
Wireless internet access
Free fax facilities
Shower facilities (only in Terminal B)
Alcoholic drinks, hot drinks and soft drinks
Breakfast from 6 to 8 a.m. and snacks
A selection of newspapers and magazines


Access to this lounge is possible for Brussels airlines Privelege status members or Business Class passengers, B-flex passengers travelling on Brussels airlines can buy access for € 15 or 3,000 Privelege miles