Using Marlin with no limit switches on delta printer

I'm working experimental delta design where I cannot yet put in limit switches - I simply manually position the axes to their initial "home" position. How do I now tell Marlin that the printer is at, say, location 0, 0, 100? Using G92 does not work - the kinematics get completely confused. I see there are MANUAL_*_HOME_POSITION parameters for X, Y and Z, but they seem to have no effect unless you actually home the axes, which I can't do because I have no limit switches yet. Everything works fine if I just declare the starting position as Z=0, but that's pretty inconvenient to work with. Is there any way to force Marlin to simply add an offset onto the G-code positions without homing?

What you're saying kinda makes sense to me. Sitting here looking at my Kossel and thinking about the steps:

1. Move the nozzle into the Z0 position which is one paper width away from the bed. Then G92 Z0 here.
2. Move the nozzle up to an arbitrary point and set home position here.

Then, when the printer homes it should go back to that arbitrary position.

That does of course require code changes.