rough running 912ULS

Mike Asked  " ... is that 5 Gallons or 5 Liters...? "

Google yourself a  "Flow Through an Orifice"  Calculator.

Assuming 5 psi and a 0.035" orifice, the result shows about 3.5 gal/h for water.
Assuming  7 psi and  a 0.020" orifice 5l/h might be a better figure.
Gasoline is less viscous so you will see an even higher flow.
Your rate will vary.
If you were to return that flow to the wrong fuel tank, you may risk an overflow.

I live in a 45 psi world so may have overstated your numbers, but the underlying warning still applies.

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I have a 912is  at 45psi that uses a restriction port similar to an orifice.
My actual measure return flow is closer to 8 gal. per hour.
My fuel tanks are cross-connected by a 3/4 hose and self equalize. 
It is such a non-issue in my particular case that only one tank has a fuel fill.
The second tank takes about 20 seconds to equalize.

Historical Note:
The return orifice first appeared on the fuel-injected engines at 45 psi.
The idea was then applied to the Carb'd engines at 5-7 psi, but the orifice size was not adjusted.
So the ULS return flow is significantly less.

I will argue for a larger orifice for the Carb'd engine.
The fuel pump is rated at over 30 gals per hour.
Bypassing even 5 gallons still leaves 25 gallons available for an engine that burns less than 5.

You could plumb your return fuel via your boost pump fuel line, assuming that you have a separate line for your boost pump.