What do you mean by this? You can’t put other eggs under her?
You might be able to sneak a few in once, but they're pretty feral about it, it doesn't take much to convince them their nest has been violated and isn't safe.
What if you put day old chicks under the broody leghorn hens before they gave up, do you think they’d care for the chicks?
I certainly wouldn't count on them to. If you want a hen to do that, I'd got for a Silkie or bantam Cochin or some such breed.
As for leghorns hatching chicks I would be very interested in seeing a closeup of a leghorn hen brooding a clutch of chicks.
I'll see if I can find some! I'm several generations and 8 years in, (and this was several computers ago as well!) and my original Leghorns (white Leghorns and California Whites, from Cackle Hatchery) are all long gone - they lived 3 to 5 years on average, and to my vast surprise, a couple of those and an EE hen were the only birds I got that every went broody. None of the Barred Rocks or Buff Orps or sexlinks ever did.
I had gotten 50 assorted pullets, with a note that unlike many, I LOVE Leghorns, so I ended up with a bunch of them, and at the same time I got a Fry-Pan Special. I raised them all free range, penned at night and found that Leghorns will happily (blissfully, joyfully) go feral and act for all the world like Games. To this day I spend a lot of time making sure everyone is in the coop, but they forage so well & produce so well it's worth it to me.
I culled the roosters as soon as they showed any traight I didn't like and ended up with a California White rooster, 2 EEs and a lovely Barred rock. A RSL (and they're Leghorn cross) rooster was well mannered enough to hang around long enough that my current oldest bird, a 7 year old hen, was fathered by one and out of an EE hen.
All of the birds I have today are descended from those, and show heavy and obvious Leghorn influence, with a couple that look EE. My eggs are fairly consistently cream colored, but I hatch out the rare green egg layer.
I'll look for my old pictures of the original girls with chicks - the little white hens can't be mistaken for anything else - and I'd be happy to show off my current mutts if it would help anyone.