Tubus swing is high attached to fork crown, to let your suspension fork work,

to use low riders you will have to hang the mass on the bottom of the fork,
increasing Unsprung weight to the suspension.

Placing the bags on the Swing racks on the steering axis worth a try.
head angle is the steering Axis. Maybe move those back?

They have a Mid fork rack this year the Smarti not real low riding, so more ground clearance
top rail attaches to V brake bosses..

Old Man Mountain uses QR skewer for its bottom mount, It may be removable ..

losing the sus fork will get the Low Rider to work, Suspension corrected rigid fork with rack braze-ons..
I have a Koga Trekking bike. they had a contract with a TW Machine shop to drill and tap the lower slider
on a particular model of Spinner Forks, and some special parts made to mount Tubus Ergo racks ,
cost effective if ordering Hundreds as an OEM ..