It looks like you're on a hiding to nothing, consider moving your tray closer to the final outlet connection. There are numerous potential downsides of too shallow a fall and a non-uniform fall, most of which have already been covered.
Chopping lumps out of 9" joists? The maximum allowed by Building Regs (Law) is 12.5% or 30mm in your case, and that's between a distance of 7% and 25% of it's span, so not enough to get your pipe below floor level.
A 40mm ABS pipe (solvent weld) is 46mm o.d., which means you'll have just 12mm of fall to play with if you use that method. {edit...ignore that last sentence, it makes no sense}
If you can drill 50mm holes in the joists (assuming they've not been notched or drilled in the same area already) then you have a little more leeway, but you must only drill the central 50% of span region (between 25% and 75% of joist span). The hole may be up to 25% of depth (60mm), wholely in the central 50% of depth (top of 50mm hole no higher than 60mm from top of joist, no lower than 110mm from its bottom). This equates to a maximum fall of 70mm, assuming your house and your joists are level, over the entire length of run, or a 1.7mm straight run at 40mm per metre fall.
In any case, and whatever laws and rules you care to break or follow, design your run so that you can rod out the pipework effectively with as little dismantling as possible. Use no more than one 90° bend in any place without a straight run to a rodding point.
If my Regs statements are wrong/out of date then I'm sure others will be along shortly to correct them.
MM