It is quite easy, actually. The parameter name is of the simulink block is "ExtGeomFileName"
load_model('simulinkmodel1')
% ....
% Only for Mac users
if ismac
% Find all the blocks in the simscape model
blocks_in_model = find_system('simulinkmodel1/system', 'LookUnderMasks','all');
% Check for all the blocks if there exists a parameter with this filepath
for ii = 1:length(blocks_in_model)
try % if this param exists, replace the slash.
filepath = get_param(blocks_in_model{ii},'ExtGeomFileName');
macpath = strrep(filepath,'\','/');
set_param(blocks_in_model{ii},'ExtGeomFileName',macpath);
catch % No such param
end %endtrycatch
end %endfor
end %endif
Easy peazy quick-fix-squeezy