We Siebel developers have used scripting to correctly determine if record is new. One non scripting way you could try is to use RuntimeEvents to set a profileattribute during the BusComp NewRecord event, then check that in the PreWrite event to see if the record is new. However, there is always a chance that user might undo a record, those scenarios are tricky.
Another option, try invokine the BC Method:IsNewRecordPending from RunTime event. I havent tried this.
For the second part of the query, I think you could easily solve your problem using a PickMap.
On Opportunity BC, when your pick Account, just add one more pickmap to pick the Locked flag from Account and set it to the corresponding field on Opportunity BC. When the user picks the Account, he will also pick the lock flag, and your script will work in PreWriteRecord.
May I suggest another solution, again, I haven't tried it.
When new records are created, the field ModificationNumber will be set to 0. Every time you modify it, the ModificationNumber will increment by 1.
Set a DataValidationManager ruleset, trigger it from PreSetFieldValue event of Account field on Opportunity BC. Check for the LockFlag = Y AND (ModificationNumber IS NULL OR ModificationNumber = 0)) and throw error. DVM should throw error when new records are created.
Again, best practices say don't use the ModNumbers. You could set a ProfileAttribute to signal NewRecord, then use that attribute in the DVM. But please remember to clear the value of ProfileAttribute in WriteRecord and UndoRecord.
Let us know how it went !