Would it be "RF back" (as per standard back lock) or "RF back & slightly rightwards" (as per V6) ?
I think you have those the wrong way round, but I may have an answer for you.
The QS forward lock changed slightly between Alex Moore's 3rd and 4th editions. In Ed3 the step is described as 'not yet standardised' and has the LF placed forward. Ed4 introduced the 'forward and slightly to side' idea, and also included a comment that the foot could be placed 'diag forward' for greater progression sideways along the LOD. The ISTD 10th ed. uses 'diag forward' while the IDTA and UKA use 'forward and slightly to side'.
If we invert this to get the back lock, then we have the "RF back & slightly rightwards" or "RF diag back" we are familiar with.
If we accept that this slightly rightwards action is to increase the travel along the LOD then this is appropriate when danced typically backing DW. However, when used as part of the V-6, or the waltz combination you suggest, we are backing DC and moving leftwards along the LOD so we remove the slightly rightwards action.
one more potentially relevant point is that the turning lock to left also has a rightwards on step 3.
In this case the sideways movement is appropriate because we are also starting to turn on this step.