The move means Welsh Secretary Ron Davies will almost certainly become First Secretary of the devolved body when the 60-member assembly is elected for the first time next May.
Scottish Labour members will hold their conference on the same day - September 19 - in what is already being billed as a " Double Coronation" to confirm Mr Davies and his Scottish counterpart Donald Dewar as leaders of the Welsh Assembly and Scottish Parliament.
The decision by Welsh party officials to hold an electoral college instead of the "one member one vote" system has been attacked by opposition politicians as undemocratic.
Mr Davies has signalled his support for a special conference to avoid internal wrangling and put a leader in place well in advance of Labour's campaigning in the Assembly elections.
His likely leadership opponent, Labour's Cardiff West MP Rhodri Morgan, wanted the issue put to every party member in Wales - a view shared by many of his supporters.
Instead, the Wales executive meeting in Cardiff "overwhelmingly" endorsed a recommendation by a party "taskforce" committee to hold an electoral college comprising of blocks of votes. Constituency parties, trade unions, MPs, MEPS and the selected panel of National Assembly candidates will be represented.
Earlier, Plaid Cymru leader Dafydd Wigley MP suggested that his party was ready to consider a coalition after next year's assembly elections.
Mr Wigley signalled that Labour support was likely to slip over the next few months as Britain's economic problems increased.
"If Labour were three to four seats short of an overall majority they would look to the Liberal Democrats as their natural partners (in the assembly)," he said.
"If there was still not enough support for a governing coalition, we would have the responsibility to see whether we could cooperate on an agreed programme."
But the Plaid Cymru leader stopped short of demanding a referendum on independence for Wales as the price for any support in the new assembly.
"We don't go into the assembly to use it as a springboard for independence. That is not what we are about or what Wales is ready to accept," he said.
He later stressed that his party's first option was to be the largest party in the new assembly, and if possible, win an overall majority.
Mr Morgan is confident Labour will win an outright majority in the assembly.Reuse content