I couldn't quickly locate anything regarding OBR elsewhere except for J/R. It may be there, but I couldn't find it in a quick search.
J/R states:
Ground Rules
Before the initial game of a series, the home team manager should specify for the umpires 3.13 and opposing manager any ground rules (which refer to areas or things which are live ball territory or dead ball territory). If such rules are acceptable to the visiting manager and do not conflict with or supersede any official or league rule, the plate umpire should concur in acceptance of the ground rules. However, if the opposing manager objects to a ground rule, or an umpire recognizes a conflict, the plate umpire should mediate the objection or conflict and, if necessary, impose a relevant solution [my emphasis]. Furthermore, a plate umpire can impose other ground rules that he believes are necessary.
Because of the threat of injury, a home team manager often propounds a ground rule prohibiting a catch in the dugouts; such a rule (for example) supersedes a book rule regarding a catch (see Chapter 2).
It doesn't state the umpire cannot circumvent the rule if agreed to, merely that he should impose a relevant solution. It appears for OBR they recognize the differences that may occur and want to be certain it is acknowledged and agreed to by both coaches before starting the contest.
Of course, Fed is very direct in that their book rules cannot be superseded by local rule. They address that with casebook example.
Just my opinion,
Freix