I'd start with sensory inputs. What sensors does the Passat have, and how do they relate to the Subaru?  Is the Passat cluster CAN?

Awesome, sounds good.  For the record, if you Google "Tuning cluster Audi/VW" on Nefmoto (use Google, the Nef search sucks!), you'll find a good thread.  You can basically tune the cluster to suit your needs.  For example, buying a diesel cluster and adjusting the scaling maps for RPM to suit a petrol.

With that in mind, you could modify the scaling for the fuel gauge to suit your sender.  What sender are you using?  Subaru?  Could you replace with a VW one?

I have an early Audi A4 cluster fitted, the first generation works with a "reversed"  sender. Meaning that you can stick with your original vanagon fuel sender without modification. If it does read in the opposite direction, take out your sender and modify it with a bridge so the signal is reversed.

Pinout for cluster 1:

26-Pin Connector, Blue
1 - Open
2 - Signal for buzzer
3 - Terminal 15
4 - Terminal 15
5 - Open
6 - Terminal 61
7 - Parking brake/brake system malfunction
8 - ABS (input signal)
9 - Airbag (input signal)
10 - CAT
11 - Open
12 - Open
13 - Parking light, right
14 - Ignition switch, S-contact
15 - Chime signal
16 - Open
17 - Open
18 - Oil pressure 1.8 bar
19 - Terminal 58d
20 - Coolant level
21 - Turn signal, right
22 - Headlight high beams
23 - Speedometer Vehicle Speed Sensor (VSS) (checking → Chapter)
24 - Brake fluid
25 - Brake pads
26 - Parking light, left

26-Pin Connector, Yellow

1 - Terminal 58
2 - Terminal 58
3 - Engine RPM signal
4 - Air conditioner compressor
5 - Clock (output)
6 - Speed signal 1
7 - Speed signal 2
8 - Door contact (driver's door)
9 - Terminal 30
10 - Terminal 30
11 - Open
12 - Fuel tank warning
13 - Turn signal, left
14 - Open
15 - Open
16 - Seat belt buckle switch
17 - Open
18 - Terminal 31
19 - Terminal 31
20 - Diagnostic wire
21 - Open
22 - Open
23 - Open
24 - Open
25 - Sender for fuel gauge
26 - Engine coolant temperature sensor

20-Pin Connector, Black
1 - Fuel consumption signal
2 - Open
3 - Open
4 - Open
5 - Outside air temperature
6 - Selector lever display
7 - Open
8 - Open
9 - Coolant level
10 - Hydraulic pressure
11 - Trip computer - Reset
12 - Radio/Telephone - Clock
13 - Radio/Telephone - Data
14 - Trip computer (forward sequence)
15 - Radio/Telephone - Enable
16 - Rear lights/headlight low beams
17 - Open
18 - Trip computer (reverse sequence)
19 - Brake lights
20 - Open

4-Pin Connector, Black
1 - Oil temperature sensor

My biggest concern is that I will have a subaru ECU.  I'm assuming I will have to graft a K line connection, which is fine, but is there anything I'm not seeing?

Also, will this cable do the trick?

http://www.amazon.com/VIMVIP-VAG-COM-409-1-Scanner-Volkswagen/dp/B00K6498D0/ref=sr_1_1?ie=UTF8&qid=1449686121&sr=8-1&keywords=kkl+cable

So how do I access the cluster? Through OBD or some other port?

This is gonna sound patronising, but it's not.  

So, you've got this old van you've put a 2009 Subaru engine in, right?  And you're going to through a B5 cluster in it?  How are you going to get RPM, speedo, oil pressure, ABS, airbag, TC, screen washer lights to work?

I think the fuel gauge is the least of your worries at this stage.  

From an aesthetic standpoint, the B5 cluster looks and fits great (A4 as well). It also has a better information interface than the shitty stock Vanagon unit. I'm also switching from KPH to MPH.

My concern about speed is that I'm still using the stock Syncro transmission, so I may have to set up a hall sensor or something, but there are numerous ways to fix that issue. Tach should be fine. It will be working with the stock unit. Oil pressure will require a a sensor, but again, fairly straight forward.

I'm hoping to add cruise and a new instrument cluster could help with that as well.

My worry is that I've only seen this done with the 1.8t swaps. So, VW engine to VW cluster. I am hoping I can communicate through the Subaru OBD to the VW cluster.