It would be great to be able to drop triplets into step sequencer patterns without having to program a new pattern based on threes.

I think you can, unless I'm not understanding your interest.
 
With the step sequencer dialog box open, select the note type (i.e. 1/4, 1/16, etc.)  Then check the box before "Tuplet" and the two fields that follow it will let you determine the note number and time amounts.  For instance, a normal 1/4 note triplet would be set by placing notes 3 in the time of 2 notes.
 
This approach is more robust than a simple triplet.  For instance, you could place 5 notes in the time of 3, 27 notes in the place of 32, etc.  The field entries are limited to 32 so you wouldn't be able to place 47 notes in the time of 48, but now we're just getting silly.
 
I think there's a process for entering this as a pattern, but I got the idea you already figured that out.

Oohh!  I'm sorry, I re-read your post and I realize now that I misunderstood your thoughts.  I was answering based on step recording.  
 

Thanks - I think a better way to describe my wish would be to be able to change the steps per beat resolution within individual beat(s) in an existing sequencer pattern.

Yup - I too have to currently create a clip based on division of beats by 3's, and it is quite the pain, when a passage has a number of triplets and non-triplets to capture.
 
Bob Bone
 

I really only use the step sequencer for drums, so by the time I'm ready to put in my fills and variations I am well and truly committed to my step/beat resolution.  So at that stage inserting a drum fill that includes a triplet (or a 5, 6, 7 etc for that matter) means programming a whole new pattern based on whatever resolution the fill/variation requires, and making it fit with the rest of the drum part.  It is possible to change the step resolution in an existing pattern, but the existing notes don't move to accommodate the new step resolution, so you have to more or less program the new pattern from scratch.
 
By the time I've done all that I've usually forgotten what I was trying to achieve by using a triplet in the first place! As Bob said above - it's a pain!
 
 

My default pattern is a 12 step per beat pattern which allows me to do tuplets, dotted's or straight rhythm patterns.  It does require a little extra thought, but it works fairly well.

I ended up with the general approach of a 12-step per beat setup too, as it is the lowest common denominator for 3 and 4.
 
I additionally REALLY desire the ability to specify an eighth-note meter base in the Step Sequencer, so that we could properly create clips for 3/8, 6/8, 7/8, 9/8 etc.  Currently, I have to double the project tempo and create clips using 3/4, 5/4, 6/4, 9/4 whatever, and then everything takes up 2 measures instead of the one it would be with the eighth-note meter base, and then I also have to insert another meter change to drop it back to half of what I had needed for the doubled-tempo for the quarter-note base.
 
Further, I have to use a project tempo that is divisible by two, in order to support the doubling and halving, and that is really frustrating.
 
Bob Bone
 

I gave up using the step sequencer for drums when I realized how much better Geist's step sequencer is. It's not perfect but it's just SO much better and I'd love to see Cakewalk update theirs taking a few pointers from Geist.