If you do a search on the BA website for "MFU", the following language will appear. I had to quote this to the agent:
"If I have a return ticket, can I choose to use Miles for Upgrade on a one-way basis only?
No, your whole itinerary must be upgraded.
The only exception to this would be, if you wanted to upgrade the entire journey but the higher class of travel was only available on one sector.
In this instance you would be allowed to upgrade on the one sector available and would be charged half of the BA Miles required for a return journey."
So, yes, it can be done; just be persistent.
PD - can you shed any light as to exactly where this language is hidden - when I searched I didn't get any hits for MFU. The only reason I ask is that a friend was trying to MFU a regular WTP return to Club on the outbound leg only. The agent (in Jacksonville) said that it could only be done on both legs. As there was no MFU availability on the outbound leg the agent said the only way to do it was to change to a flight that had MFU availability (at a cost - change fee plus fare difference!) and upgrade on both legs. Seems a bit extreme but the Ts and C's that I can find on MFUs state that one way MFUs are only available on single tickets:
"3. One way upgrades are only allowed on one way bookings."
This language can be found if you go to the "Using BA miles" link then to the "Book and Upgrade with BA Miles" tab and then click on "Miles for Upgrade conditions of use" link.
https://www.britishairways.com/trave...club/_gf/en_gb