Casey Brown Named Head Coach at Penn

Does the PL have any constraints on endowment coaches positons?

All you need is enough money to endow the position.

Don't know, but I'd guess at least 20x.

$2.5 million endowment at a 4.5 percent distribution would pay a $112,5000 salary. About 23x.

To grow the endowment so that the distribution would cover increased salary in the future,  one either hopes smart investing grows the endowment value by more than 4.5 percent annually, or a small portion of the endowment value is reserved for re-investment and not distributed.

I wonder why I have never heard of a solicitation from HC to endow a coaching position. Depending on which sport is targeted, such a campaign might be successful. I know that donors may designate which sports are to receive a person's donations, but that's not the same thing as a dedicated endowment for coach salary.

Endowed coaching positions are typically funded by alumni who played the sport, parents of students who played the sport, or an alumnus / benefactor with a lifelong interest in the sport.
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Tommy Amaker is the Thomas G. Stemberg '71 Family Endowed Coach for Harvard Men's Basketball. Stemberg died in 2015 at the age of 66. The story of Stemberg's association with Harvard basketball is narrated by his widow here:
www.gocrimson.com/sports/mbkb/2017-18/releases/20180222m64id0

I bring up Stemberg because he is a member of the Cornerstone Society at Holy Cross.
Scroll to the last page.
www.alumni.holycross.edu/s/1380/images/editor_documents/presidents_council/pcdinner2013/pc_program_final_web.pdf?sessionid=0e0b7b2c-8324-46d1-80a0-797433524b24&cc=1

His generosity was in memory of a good friend, a Holy Cross alumnus, whose name I can't remember but who may have played basketball. AFAIK, the gift did not go to the basketball program. I believe his support of Harvard basketball, during his lifetime and posthumously, was directed funding for Harvard basketball.