You would never want to discharge fines into an attic (or any other essentially “enclosed” and uncontrolled space — the waste bin is a controlled space of limited volume).
It is important to understand that not only are fines highly flammable (any flammable substance with a large surface area relative to its volume is high risk as the greater the surface area the faster it will burn) but are also potentially very explosive if they get airborne and encounter any sort of spark or flame (including self ignition from dust-to-dust static discharge) — injecting a lot of fines over time into a room/attic is a bit like discharging a large mist of gasoline into that same space (only gasoline at least evaporates if it does not ignite), you may get away with doing such many times, but you are seriously rolling the dice each time. While you may think in terms of there only being a small amount of fines airborne at any given moment, over time the whole area around the attic will become coated in those highly flammable fines, which can lead to fast spreading “ordinary” fires at best, but with every surface getting coated in such fines over time anything that disturbs the space such as strong vibrations shaking it down (a large truck driving by, a minor earthquake, etc.) or a severe storm with strong wind gusts can potentially stir up a large quantity of fines where they can then remain airborne for many hours before resettling and if any ignition sources are encountered and the ratio of dust to oxygen is favorable then a serious explosion hazard exists.
In workplace accidents such events involving fine dust are often precipitated simply from an event knocking or shaking fines that had accumulated on overhead beams and rafters allowing accumulated fines to suddenly rain down upon the space (such as may happen in an attic). Oftentimes in such accidents the initial explosion event is usually very minor, but the concussive force then dislodges and drives airborne all the rest of the fines leading to a secondary and vastly more devastating explosion a fraction of a second later that is ignited by the remnants of the first explosion.
So, no, it’s definitely not something that you would want to consider doing. Either find a way to duct them outside and well clear of the building or install an appropriate bag or cartridge filter on the exhaust side to capture the fines and return the air to your shop. If this seems a bit dramatic it is only to emphasize the dangers associated with fine dust as to most of us dust seems like little more than an objectionable nuisance rather than a potentially devastating hazard to home and health, but even something as seemingly mundane as dust can be dangerous when it accumulates within an uncontrolled space over time. Like many risks, it will not bite one every time and one may get away with such, possibly even for their lifetime if fortunate, but every time is a roll of the dice and you never get to know ahead of time when your magic number will come up until it’s too late! As such it simply is not worth the risk.
