I never realized...
The Events & Occasions and Recipients navigation in the left hand column on some Zazzle pages are actually "categories" as per their RSS feed definition. This means you can use them as another way to specify what you want in feed requests.
As it works on feeds, so it will also work on Nifty Slideshows (and PinAProd, SeeMyBest and others).
Do you see that &pce in the address bar? The number that comes after it is the number to use for the rss feed's cg= thingy.
That means you can use it Nifty Slideshows. Here's one for the encouragement category 103972392227461916:
And here it is in place on the Nifty Slideshows generator:
It's the one after "Limit results to this category number" and you can see one of the products selected from that category (with search "new journey") in the slideshow preview at the top.
Neat :)
I've only shown it for the Events & Occasions navigation panel but it works identically with the "Recipents" nav in that left hand column (top pic). It'll appear as the number after &pcr= in your browser's address bar.
Shame we can't use both at the same time like you can when navigating on Zazzle :(