Dear Carrie, another terrific article! Gardening involves thought AND effort (a lot of both). Depending on what roses you have, your thought about encouraging branches to grow horizontally (versus vertically) is correct. Somehow it "fools" the plant into sending up buds along the branch not just at the top of it. This is, of course, more easily done if the rose is growing alongside a wall or fence, where the appropriate support can be given to encourage this growth pattern. Being a person prone to "jerry building," when these roses are not growing against a wall or fence, I wonder whether you could tie fishing line to the longer branches and force their horizontal growth with sinkers? The line would be almost invisible and the sinkers themselves could be hidden beneath some ground cover plants. The best option is to have sited them correctly the first time, obviously. However, how many of us have not spent sufficient time in "thought" and then have to compensate with "effort?"
I planted them on a trellis to begin with, it just never dawned on me that there was anything more to it than that. If I had only known to begin with ... I would have nice, short, horizontal, blooming, climbing roses.
I took a rose growing course with an ARS rosarian early in my rose hobby and was told about the benefits of placing canes horizontally. It makes a chemical rise to the top of the cane and that chemical stimulates the growth of vertical canes. I have a couple climbers and ramblers wrapped horizontally around a few dead tree stumps and big portions of the canes are horizontal but don't touch the tree. Those parts produce the vertical growth without touching the tree so there might be a way to concoct enough support without a wall or trellis. Just a hunch.
The other thing i've tried that works is placing rows or heavy duty wire hoprizontally along a 6 foot wood fence at different heights. I attach the wire to gutter nails, and make sure there is a least 4 inches of space between the wire and the fence so the branches can get enough air circulation. The rose goes at the base and can be slipped behind the wire as it grows, or tied on to the front of the wire. Very cheap substitute for trellising.