I think the NULL srid is the simplest way to go. I mean you could go to the effort to create a custom projection that forces simple planar handling but it wouldn't treat the geometry any different than NULL. Oracle Spatial was designed to provide seamless handling of geodetic data. It's not uncommon that folks using Oracle Spatial are oblivious if their calculations are happening on a spheroid or not. So there is no special switch that I know of to force planar handling other than using a NULL srid.
Now the duct tapey bit might be that once your records have a NULL srid you can no longer execute SDO_CS.TRANSFORM against them. But then you know the original SRID and can always manually swap it back if you need to transform the data.
I think you are saying that the originators of your data decided back in the day to use a planar interpretation of geodetic coordinates to define and manage their dataset. That may have been a poor design decision - it would have been better to choose a nice appropriate projection for the area or just go all geodetic. I think that is where most of the negative feedback to your question comes from and I don't necessarily disagree. It seems duct-tapey simply as the designers of Oracle Spatial never thought it a good idea to do this. They'd have said that you have geodetic coordinates representing real world locations on the globe and why would you then not want to calculate on the spheroid? But I also understand we often have to deal with what is given.