I think the naysayers are being a little pessimistic. I remember the early howls that S&S couplers were obviously unsafe and would come loose while riding. Ritchey's system just uses one tiny clamp and the seatpost and still seems to work.
The angle cut on the Herse downtube allows the top tube to telescope (it isn't a straight butt fit). The downtube has a long external sliding sleeve. It sounds like your system could do with a little more contact area between frame halves.
Apart from the Herse demountable, there was a similar retrofit system in the late 1970s or so (Pingle or Pengle, from vague memory) that didn't seem to have too many problems (noted in Bicycling! magazine). The sleeves may have had pins in addition to clamps. I wouldn't fit it to a thin gauge frame though.