Looks like Agnes sometimes appears when you re-enter a cleared room; and _sometimes_ offers a stat increase item.
Is stat farming possible, or are the amount of items offered limited?

EDIT: After a quick test this is either slow or not so straightforward. After alternately visiting two Agnes locations a few times, she either offered a magic potion, a black onyx, or apologised for having nothing. Either the stat improvements are pretty rare; or they have some condition to appear (maybe once per level, or maybe only after touring several Agnes locations?)

Gaining a level doesn't seem to make a new stat item appear; nor does progressing the plot.

My guesses at the minute:
* it depends on level but doesn't reset every level
* it resets when you beat a boss
* it's simply really rare
* it depends on you touring all Agnes locations

On that last point, I'm not totally sure where she shows up. I know of:
* the waterfall in Clover Ruins
* the exterior walkway in Egret mountain entrance
* the long chamber near the start of the Xanadu labyrinth.

I spent some time backtracking and can't find others. I thought she appeared near the first entrance to Ancient Path 1 but I can't get her to appear again.

Looks like the answer is here; there's only 1 item offered per location Agnes appears at. Oh well.

http://falcom.xfu.jp/wiki/wiki.cgi/xanadu?page=%BD%D0%B8%BD%BE%EC%BD%EA