The problem in a nut shell was that on the test versions I detected no change in the Misc Debug Params when plugging / unplugging USB in shooting modes. In play mode I did see a difference.
If you'd like to investigate further:
Start with clean CHDK source, apply nafraf's debug patch, uncomment the following likes in core/gui_osd.c:
// sprintf(osd_buf, "0xc0220200: 0x%08x", *(long*)0xc0220200);
// draw_txt_string(10, 7, osd_buf, conf.osd_color);
// sprintf(osd_buf, "0xc0220204: 0x%08x", *(long*)0xc0220204);
// draw_txt_string(10, 8, osd_buf, conf.osd_color);
// sprintf(osd_buf, "0xc0220208: 0x%08x", *(long*)0xc0220208);
// draw_txt_string(10, 9, osd_buf, conf.osd_color);Make builds for those 2 cams, watch the numbers after 0xc022... . If you don't see a change there when plugging in/out USB, adding support for USB remote will be probably harder.
Anyway I marked the IXUS 30 and IXUS 40 as USB Remote completed on the overview page
I have
committed that change.
edit:
The ixus700 uses the same GPIO line as the ixus30/40 for sensing USB. Since I don't know how individual GPIO states get grouped into the 0xc0220200...0xc0220208 registers, it may not show up there.