Good clarification Norm. Some boxes do have needles without an amp, like the C-100F, they just tap the preamps with a diode and resistor. That was sort of my line of thinking. Is it possible the needles on those units have a different type of sensitivity to allow that to work without an amp?
But Dancorp, your point about looking at the service manual for a close relative is a good idea. I found one for the RS-4300 here
and although it only has a single needle and additional shortwave bands, everything else is essentially the same. The user manual here
mentions the switchover to show audio during recording, like your RS-4150 does. "The recording will begin if the record button and playback button are pushed at the same time. The indication needle of the level meter will fluctuate during the recording depending on strength of the sound."
If you can determine that you have the same circuit it may be possible to figure out which switch sections on the record bar are responsible for providing audio to the needles during record mode.
The first page of the RS-4300 schematic is the right half of the sheet, and page 2 is the left half. If you look at the meter on page 1, you can see all of the lines for the various inputs (mic, tape, radio, aux) all passing right above it from the left half the sheet (on page 2) and converging on an area to the right of the needle, going through some resistors.
The 4300 has a single three-position function switch, S2, for (T)ape or ®adio or (S)leep, Sleep is radio only, you just play or record a tape until it stops for your "timer." It looks like the 4150 has a separate tape/radio switch to allow you to fall asleep to tape or radio.
Section 12 of the record bar, S1, toggles the needle between battery/tuning in playback and audio signal for recording, but i'm not sure exactly how it does this, because it looks like it just runs 9V through R204 and TR21. Battery level just comes straight through R206 normally. If you can determine that you have the same basic circuit and two sections of the record bar indeed tie directly and only to the needles, you could try cutting and jumpering them if you're feeling adventurous.