Someone who occasionally proofreads for Mises.org recently double-checked with me that it’s OK to add an apostrophe s to form the possessive "Mises’s."

The short answer is yes it is OK and it would violate house style to form the possessive in any other way.

Here is the longer answer:

Not only is it OK, but it would be wrong to have it otherwise — not only in our house style but in general according to what the Chicago Manual of Style refers to as Strunk and White’s famous rule 1 ("Form the possessive singular of nouns by adding ’s"):

William Strunk, Jr. (1869–1946).  The Elements of Style.  1918.

II. ELEMENTARY RULES OF USAGE

Form the possessive singular of nouns with ’s.   Follow this rule whatever the final consonant. Thus write,   Charles’s friend Burns’s poems the witch’s malice   This is the usage of the United States Government Printing Office and of the Oxford University Press.   Exceptions are the possessives of ancient proper names in -es and -is, the possessive Jesus’, and such forms as for conscience’ sake, for righteousness’ sake. But such forms as Achilles’ heel, Moses’ laws, Isis’ temple are commonly replaced by   the heel of Achilles the laws of Moses the temple of Isis   The pronominal possessives hers, its, theirs, yours, and oneself have no apostrophe.