Hanging kitchen cabinets on 2x3s
No code issues at all.   In fact using 2x3s for interior non-load bearing walls is a staple of &quot;cheaper&quot; construction.
However yours being next to a cinderblock wall negates the cheap aspect (cheap because when you are in a house with 2x3 walls, they often seem &quot;shaky&quot;).   I would just make sure the 2x3s are of high quality because when you start getting into smaller dimensional lumber you can get some warpage that makes getting a straight wall up harder.
If that inch is helping you that much you have good reasoning to use 2x3 and given good/straight lumber you wouldn't notice a difference.  I will also say if hanging cabinets it is a good idea to run many cross braces in the framing to help support.   If you run two lines with the board flipped at strategic locations, installing cabinets a breeze.