First off it works over both letters and objects but differently!
I this first example its the middle layer
I took the rectangle with the gradient in the background and then ONLY then applied the filter and then reduced that rectangle in size and then moved it back over the bottom layer all in Inkscape. The lettering was created separately as a final layer .
One of the reasons I created each layer separately was because of this experiment. Applying the filter directly to the text turns the whole text to leaves however if you create a duplicate scatter leaves over that and then do some adjustments with ... its closer to the effect I want
I also tested Scatter leaves over an Object generated by Unicode the Hanzi / Kanji for Autumn . My first attempt created a pretty pattern but didnt work placed over the hanzi so I used it to create a background file and then I retried Scatter leaves over a Square with a Gradient again with the kanji lowered.
I also tried this header which I found looked better with the bottom layer being a text with a very simple gradient but a different gradient with more steps was used on the duplicate that became the top layer.
The major problem with the Scatter leaves filter seems to be that if you apply it on too large an area or object the leaves just look like blobs of color however used in small areas or as a background you can get some interesting effects especially combined with the use of Gradient. Expect to do a lot of tweaking of the Gradient though. Edits of the Gradient took the most time for me.
Some of these I will refine further or redo. One idea may lead to a print with an autumn leaf haiku or waka on it.
I hope I've shared some interesting ideas that may inspire your own better work?