We have very similar grills. In fact, yours is a down-market copy of mine. Mine, in turn is a down-market copy of the Fire Magic.
First -- forget the hood thermometer. It is a cruel hoax.
You're right about how well this style of grill cooks steak. There's only one other style I know of which does as well. FWIW, what you're doing for steak is what you want do do with fish.
You're right about how useful it is to have a grate with no fire under it. You'll find that's the best setup for chicken, too.
There's another, similar style you'll want to experiment with that's useful for big pieces like beer-can birds, tri-tips and 3 pound hunks of sirloin. Remove the cooking grates, open the belly door, raise the charcoal tray. Build a large fire in the center of the grate. Just before the coals are quite ready to cook, bank them on each side of the tray with a shovel. Then move a few coals from each bank into the center of the tray. Your fire should look like ^__^ This not quite indirect method is good for a lot of things. Working with a slower fire, you can even do a good job on ribs.
You're right about the bottom vents sabotaging "low and slow" cooks -- but only to some extent. Yes, you can block some of them with aluminum foil or construct sliding dampers, for that matter. The problem isn't the lack of a chimney, either. Unfortunately, that grill will never be a good smoker. It's simply too drafty and too hard to control low temperatures. When it comes to "low and slow," or even medium indirect, our grills can't begin to keep up with a kettle type. There are a few console types that can (Cajun, Char Broil 940x), but we don't own them.
That having been said, the best way to control lower temperature fires is with fire size and type of fuel. (Briquette burns cooler than lump). Open doors only when absolutely necessary and get them closed ASAP. Use a moderate sized fire (1-1/2 small chimneys or 1 Weber-sized chimney). Double banking with a drip pan (with some water in it) underneath the food is better than a single bank of coals. Hood vents barely open. Leave one cooking grate off, center the other grate(s), and add fuel as necessary from the open spaces on either side.
I suppose with enough metal work and modification, you can make your grill work like a smoker. With enough time and care, you could probably do it without compromising grill performance too badly. But why? At the end of the day, the Brinkmann is an entry level charcoal grill. The porcelain-over grates will chip and the pantograph charcoal tray adjuster will die. 2 years if you don't take care of it, 5 if you do.
We own the grills we own. It sounds like you are as happy with yours -- as a grill -- as I'm with mine. But the truth is, the only way either of us can approach real smoker performance is with (steady, wait for it) a real smoker. There are some good options for ribs, pulled pork, and brisket in the $30 - $300 price range, including the Old Smokey at the low end. The Weber Smokey Mountain aka WSM (bullet), and the Bar B Chef Offset (small patio offset) are the performance stand outs of their types.
If you can afford the $200, I think you'd really like the WSM. They're as good as anything else you can buy at any price. Plus, they're very fuel-efficient, require little tending, are easy to learn and double as good portable grills, You'll use your WSM long after you outgrow the Brinkmann.
Rots o ruck,
What were we talking about?
Klose Steak Grill with Swing Set
Backwoods Fatboy with DigiQ II