the FIA rules do offer the option to change a rule if all competitors in the series agree.
this is in the FIA Rules and Regulations, in the International Sporting Code, article 199 amendments to regulations, regarding sporting rules and regulations: i point to point d, item c.
Changes to sporting rules and to all regulations other than
those referred to in b) above are published at least 20 days
prior to the opening date for entry applications for the
championship concerned, but never later than 30 November
each year. Such changes cannot come into effect before
1 January of the year following their publication, unless the
FIA considers that the changes in question are likely to have
a substantial impact on the technical design of the vehicle
and/or the balance of performance between the cars, in
which case they will come into effect no earlier than
1 January of the second year following their publication.
d) Shorter notice periods than those mentioned in b) and
c) may be applied, provided that the unanimous agreement
of all competitors properly entered for the championship or
series concerned is obtained.
http://argent.fia.com/web/fia-public.ns ... v08ANG.pdf
so if the teams would have agreed on MS testing the F60, then according to the rules FIA would have agreed too.