Jim,
A couple of your statements intrigued me so I went back to look at some numbers.
In the manual for my
BX2200, I looked on the specifications page at the specification table and found the max. lift force at the lift points to be 1210 lb. of force. When I checked the Implement Limitations page I found the W0 (that's a zero) (lower link end max loading weight) to be the same: 1210 lb. Going back to the specification table, the max lift at 24" behind the lift points is listed at 680 lb. of force. (This would indicate that an attachment which is 4' long, and of equal weight distribution throughout its mass, therefore having its center of gravity at a point 24" behind the 3ph, could have a max. weight of 680 lb. and still be liftable by the hydraulic pressure to the 3ph.) It is really curious that the max. force at lift point matches the W-zero at 1210 lb. Now, in looking at the Implement Limitations page the W1 (implement weight limitation) indicates that the reader should refer to a chart of specific implements on the following page). As you state, the heaviest implement listed is 375 lb. I got to thinking, "Why do the first pair of numbers match at 1210 lb., but the implement weight of 375 lb. is so much lower than the 680 lb.?" I think the answer could be that those implements for which weight limitations are given are implements which are used while the tractor is in motion. If the tractor is moving over bumpy terrain, there is added force which is placed downward on the lift arms. A 375 lb. implement which is being bounced up and down could generate more than 680 lb. of force (at a leverage point 2' behind the arms), causing the arms to fail or the hydraulic pressure to be overcome. Now, how much cruising around would one do while a cement mixer is running on the back of the tractor? You might move your location, but you'd do so quite gingerly to start with because you wouldn't want to be sloshing your mud everywhere. I would therefore think that when the tractor is sitting still, experiencing no bouncing, that 680 lb. is much closer to the weight you could load onto the 3ph. And that's assuming something 4' long. A shorter implement with a center of gravity less than 24" from the 3ph could weigh even more.
Now, for the second math thing you got me thinking about. You stated that you had found no 3ph PTO driven cement mixer smaller than one-third yard. A cubic yard is 3 ft. to the third power: three cubed: 3 x 3 x 3, which equals 27 cubic ft. per cubic yard. The mixer for which I gave a link above is a 5 cubic ft. capacity mixer. 5/27 is a much smaller fraction than 1/3, it is in fact almost half the size. A 1/3 cubic yard mixer would hold 9 cubic ft. as 1/3 x 27 = 9. 5/27 as a decimal rounds to 0.185 cubic yd. 1/3 as a decimal = 0.333... cubic yd. The Northern Tool mixer is only the tiniest bit more than half the capacity of of the mixer that you said was the smallest you could find.
Finally, in thinking about both of these sets of numbers, I believe the Northern item is light enough to put on a BX. The unit weight of mud will vary depending upon its ingredients and proportions. The mixer weighs 215 lb. 5 cubic ft. of water weighs 343.2 lb. That totals 558.2 lb. I'd guess mud weighs more than water by a bit, which might bring it close to the 680 lb. (Still, this thing is not 4' long, so you still should be able to go more.) To be on the safe side, you could load the thing less than full.
This still leaves the questions of the width and height of the arms. It sounds like there's a hydraulic tilt mechanism which would make pouring o.k. Just from the look and the specs given, I'm thinking the mixer might well fit within the arm span.
I think there COULD be a possibility for this thing to work.
Your critiques and responses are welcomed.