I think I am the one who suggested this arrangement. If you look at Arkel's website
, you will notice that the T-42 are about as high as the GT-30 or the TT-38 (i.e. their largest front panniers). The main difference is between Arkel panniers is that hooks for rear panniers are 7" apart, while hooks for front panniers are 5.25" apart.
BTW, although I would not recommend you re-engineer their panniers, if you need to change the spacing between hooks, it is possible with a drill to drill a new hole through the fabric and plate.
What should you check?
1. On my front racks (Blackburn Lowriders), I don't have any problem installing the T-42 panniers. The horizontal "platform" is long enough so I place the hook onto one of the safety tabs of the rack. I don't think Blackburn Lowriders exist anymore, but there are lookalikes in existence (REI probably sells them). If you get another one, such as the excellent Tubus, check the length of the platform (7 - 7.5 inches). The Jandd extreme are very long racks, but some people question their solidity. As I haven't seen them, I can't help you on that subject.
2. Check that any panniers you buy for the front has a locking system. A rear pannier that falls may cause you to skid and fall. But a front pannier that falls will
cause you to fly over your handlebars.
Likewise, make sure the hook that goes at the bottom works ok with your front pannier. Some panniers just have a strap that goes around the ear of the rear pannier; as there usually is no ear at the bottom of a front rack, a mere strap isn't secure enough for the front. The T-42 are OK on these aspects, but I have another set of rear panniers (Le Montagnard) that wouldn't fit on the front rack.
3. Some rear panniers have hooks, places to hang things outside them, a platform on top, etc. This is great because it let you hang extra supplies on top of the rear pannier, but loose straps are very
dangerous around the front wheel.
Regarding roads vs how low they go
On the lowrider rack, the T-42 go down to about 6" off pavement. That's about as low as would go their GT-30 and TT-38 front panniers. But as they are a bit wider at the base, you have to be careful if you like to hug curbs (panniers will catch). If you ride or stop more than 6-7" away from the curb, you will be OK.
In tight curves, I don't think you can tilt the bike enough to drag.
Finally, rough roads, gravel roads and well-groomed trails are not a problem. If you want to ride through mud, technical terrain, forestry roads with tall hay, deep snow, then the panniers will drag.