Hi Greg.
Back to the LNAV.

It all depends on what functionality you want to get out of the LNAV.
1: Vario only
2: Vario + Wind + Final Glide with manual arrival altitude
3: Vario + Wind + Final Glide with automatic arrival altitude

For #1 you only need the LNAV. No GPS needed
For #2 you need the LNAV + a GPS source that provides Location and
Waypoint info (NMEA GPRMC and GPRMB sentences)
For #3 you need the LNAV + a GPS source like above + a source for the
Waypoint Elevation info (proprietary Cambridge PCAIB sentence)

Obviously the complete LNAV system solution (LNAV + GPSNAV20 + 303
display) provides the full #3 functionality.
If you cut out the 303 display, you cannot select a waypoint that the
LNAV needs and you can only achieve option #1
Same if you cut out the GPSNAV. Only option #1 is left.

The previous owner of my ship decided to go with and LNAV only (no
GPSNAV, no 303) + Garmin GPSMAP195 for the moving map display, when he
took delivery of the ship in the late nineties.
The GPSMAP195 is a huge brick and a huge power hog in the cockpit.
So, I removed that immediately and replaced it with a IPAQ310 (which I
am very happy with)
However, this left me without a GPS source for the LNAV.
I decided to hook up a small Garmin Etrex, which has the possibility
to output the required NMEA GPRMC and GPRMB sentences).
The Etrex lets you select waypoints, but it does not provide Waypoint
Elevation (NMEA PCAIB sentence) for the automatic arrival altitude
So that limited me to option #2.
I build a small Microcontroller application that actually adds the
Waypoint Elevation (PCAIB) sentence to the stream.
Now I have the full LNAV functionality with the Garmin Etrex (without
GPSNAV and 303)

If you (or any other LNAV users) want to have more information on
that, you can contact me offline.

I personally have no need to secure logging. My IPAQ310 does all
necessary logging for OLC uploads.
I will be installing a PowerFlarm as soon as it comes out.
My PF will be feeding GPS data to the Trig TT21 (for ADS-B out) and to
the IPAQ310 (because the internal GPS is not great).
But my PF will not be connected to the LNAV.

Conclusion:
- There is no purpose to connect a logger to the LNAV if the logger
does not let you select a waypoint (no GPRMB sentence).
- If the Logger does let you select a waypoint (output GPRMB sentence)
but is not Cambridge, you can achieve only option #2.
- If you want full LNAV fucntionality with a Non-Cambridge GPS (that
lets you select a waypoint), contact me off-line.

Hope that helps.
Hans Van Weersch