Tone-wreck,
Here I am in Brasília, but I really can't really recommend from personal experience any B&B or pousada - and there are quite a few. I am not trying to write that they are bad, not at all, but I plan to give you some ideas and need you to know that my knowledge is actually limited to details.
Anyway, first of all, you must consider that the traditional hotels, both the more expensive ones and some economic ones, were built in 2 specific hotel sectors, at the beginning of the North and South wings in the center of the city. These are great daytime locations as you take a few steps and start seeing some of the best known sites like the TV Tower, cathedral, National Theater, etc. However, at night, after the Patio Brasil and Brasilia Shopping close, there are few people meandering in these areas. The "action" then is usually is the commercial streets in the residential areas. So, most probably you would have to take a taxi to the bars and restaurants if you like to go out after 10 p.m., no matter where you stay. By a recent city ordnance, all activities in most bars (except the few that have isolated acoustics) stop at 1 a.m. weekdays, 2 a.m. weekends, because there was a lot of complaint from the residents.
If you plan to stay in this area, then I would suggest you check out the very recently opened Naoum Express Hotel. It's a very practical location, close to the Patio Brasil shopping center and a few hundred meters from the Eixo Monumental, avenue on which are located most of the well-known tourism sites. It's in the Asa Sul (south) side, which also has the some different architectural sites like the Dom Bosco church, the Buddhist temple and the Templo da Boa Vontade, an ecumenical "church" that has been attracting many tourists.
There are cheap hostels on the W-3 avenue in Asa Sul which are convenient and are used mainly by those who come to Brasilia in search of a govt job via public tests, which occur almost every weekend. Just google Pousadas Brasilia W3 and you'll get a list. These are all in converted homes or small apt buildings and are usually run by some lady - but don't romanticize these places like some cool European B&Bs.
Last, but not least, I am really a great fan of the Ruy Ohtake designed Brasilia Alvorada Hotel (until last week it was called the Blue Tree Park and Towers Hotel). That is in itself one of the locations where tour guides like to take those interested in architecture. This is not a cheap hotel and it is located a few kilometers from all the others, but it's at a beautiful lakeside location (there is Quality Lakeside and Brasilia Palace next door, which are a bit cheaper, if I'm not mistaken). During the day there are free shuttles downtown and at night you'll have to take a taxi anyway, so check this hotel out, too.
Depending on my personal and family situation and agenda at the time, I'll be glad to at least have a beer or coffee with you guys when you come. Send me a PM.
Boa Viagem!