Looking at the .SCP that the HxC software produces it is clear they are setting the ID byte incorrectly. Byte 04 was set to 14 and it should be 40. I used HXD hex editor to fix this and the .SCP file now comes up as TRS-80. There is still another issue though, Byte 08, Bit 0 is set to 1 to indicate the index should be used but the SCP software seems to ignore this. There may be some other config bit that I have overlooked though.
With Byte 04 fixed it is easy to set it to use the index and this will not erase the loaded file.
With Byte 04 fixed it is easy to set it to use the index and this will not erase the loaded file.