i am a bit late to this party but,
if you are using this technique of "aging" wooden surfaces, use all protective measures you can. Full body suit, respiratory protection, gloves, goggles and the best ventilation you can get.
While the result looks really good, the process is extremely unhealthy even with an hand wire brush. Motorized hand-held equipment makes the situation even worse.
The dust of burnt wood you are kicking up is so fine, it's clogging the pores of your skin, is crawling under your mask and again, is extremely unhealthy. Working a coal mine is a joke compared to brushing burned wood.
Have had the "Luck" to work on a project a couple years back that had wood burning/ brushing involved.
Next time someone ask me to do something like that, I'll turn around and walk away...
Even though i don't remember the details, a couple years back i read an article about cleaning up burnt forests. Not sure who brought it up, the author or the cutting crew, there was a major concern about health issues as well related to the black dust...
If you decide to use it anyway, you can skip the linseed or any other chemical treatment. The burnt surface will protect the wood from the elements better than anything. It's a ancient technique protecting wood, that has been used for ages.
Be safe. Venice