I'll caution that I'm not trying to scare anyone away from trying this, only to point out where a simple difference in dimensions or design could make or break the whole project.
If you break down what's really happening in a leaf spring suspension, the front half of the main leaf is, among other things, a trailing arm. The rear half of the main leaf does a little less than 1/4 of the side to side locating work (due to the two bushings vs. the front's single bushing), but no front to back location since it's bolted to the shackle.
So a single trailing arm from each front spring hanger could work. Though there are a couple things that need to be kept in mind.
The main leaf has some arc in it. That allows it to act as a spring in the lateral direction as well as in the vertical direction. This has the effect of softening any hard hits (pot holes etc.) that would eventually impact the front spring mount. With a trailing arm you only have the compliance of the bushings to soften these hits. That may be enough, I'm not sure. I personally would use rubber rather than urethane bushings.
The other thing is that the leaf provides lateral location. The shackles couple with the front mounts via the main leaves to resist lateral dislocation of the axle. With a trailing arm you don't have that coupling and the amount of lateral dislocation possible could increase substantially depending on the rigidity of the fixing of the trailing arms to the axle.
One solution would be to not try to rigidly couple the trailing arms to the axle in the lateral direction and to use a trac-bar/panhard bar/whatever you want to call it instead.
Which opens up it's own little can of worms. The height of the trac-etc. bar, where it crosses the trailer's chassis centerline, determines the Roll Center of the suspension. A line drawn through this point and the center of the hitch ball is the Roll Axis. This is the line in space, the "hinge pin", that the frame pivots around the suspension. The further this line is away from the Center of Gravity of the trailer, the more the trailer will lean on a side hill. So, the bar wants to be as high as is reasonably possible.
Now something to keep in mind, but do not let it drive much of any decision. More just so that some of the dynamics involved are understood rather than something to worry over. Mixed springs, meaning more than one type of spring employed on one tire, can be a very difficult thing to get damped correctly. Were we talking some sort of race vehicle (of any sort) I would caution highly against doing this. The damping that a torsion bar needs is far different from the damping that a leaf spring needs, and both of those are far different from the damping that a coil spring needs. A trailer's damping needs aren't as refined, so a mixed spring set is much less of a damping concern.
