Hi,
Ideally speaking, this can be achieved by mapping that particular plant (for which different GL assignment is required) to separate valuation grouping code because function of valuation grouping code is we can post to different GL account for each plant within same Chart of accounts by grouping the respective plant to separate valuation grouping code as shiown below where 0001 is valuation grouping code,
But it is not advisable to map plant to different valuation grouping code in production server.
Instead Is not possible for you to create separate settlement profile for this particular order type and make Settlement receiver GL account as mandatory and assign that profile to that order type . Then Try to give the desired GL in settlement rule of that order and save and do settlement and check whether system pick up this GL from the Settlement rule.
Right now i dont have provision in my system to check this.
Hope this helps
Thanks
karthik