>I'm wondering if skimming the foam would even slow down the
> fermentation by a noticeable degree.

I'm not sure I understand the goal; if you want some of the yeast for
later this is one method, but getting it and keeping yeast healthy are
not the same thing. This is a pretty tricky operation from what I have
read.

I'm a novice beermaker but I know keeping yeast from prior fermentation
is not uncommon with beer makers. Their ingredients are available all
year long so if they want to do batch after batch they really don't
have an issue with this. There is a limit to how many times they do it
though.

Wine is different in the sense that for the most part you are
fermenting it over a narrow window of time unless you use kits or
processed material of some sort.

Beer is heavily influenced by the yeast where wine is less so,
especially if it's aged any length of time. I guess that is why they
are more apt to do this than winemakers; if you are using grapes there
is already yeast on them in the bloom, that powdery looking coating on
the skins.

Joe