I'd personally get onto an equine nutritionist(independent of feed co) myself, as you want the best for her & the foal.
She doesn't need more feed above maintenance now or in the very near future - pregnant mares don't need to be any more overweight than any other horse & evidence suggests this can make not just the mare but the foal more prone to metabolic probs in future. So a normal low starch, high fibre diet that keeps her in reasonable condition is fine. I would however give her better nutritional supplementation than just a block, as they don't really provide all that much.
I have just looked for an article I just recently read by an equine nutritionist specifically on nutritional & feeding requirements of pregnant mares. Do you think I can find it, or even remember where it was from??
I think I might have got it from Dr Elanor Kellon(well worth looking up). But the gist of it re feed (can't remember nutritional details) was that horses don't need much extra calories through most of the first trimester, do need extra in the second & third trimester, but they tend to be fine on a maintenance amount for their weight in the 4th trimester. Lactating mares do need substantially extra energy to maintain health & good milk production in the first few months after the foal's birth, but their milk is richer & better on good quality forage/low starch than it is if fed grain & such.
Brewers yeast is a supp that's high in potassium, protein, folate & B vits. Can't find much info ATM on 'Stock & Stable' & you don't say which one, but as it's an 'economical across the board' type feed, I suspect it's not the greatest option & wouldn't feed sweet feed or grainy stuff to her. Can't find much(the page won't open) on the Mare Plus, but this is a nutritional supplement from Smartpak? If so, I think they generally seem to have good quality, well researched products.
Check out Sky's 'Want to share this article' thread in hoofcare section. FeedXL.com is another great online resource that I highly recommend.