I hate to complicate the matter, but I think the answer is "it depends." An ERISA expense account held within the plan can only be used for the reasonable plan expenses for that plan. However, a few years ago, the DOL issued some guidance that indicated it was OK to hold an ERISA budget account OUTSIDE of the plan assets. That is, in essence, that the service provider is providing a credit to the plan sponsor for it's use to offset plan expenses out of the service providers own pocket (with funds derived from the fees it charges, or revenue sharing it receives for that plan). Now, I disagree with the DOL on this one - because frankly, if the money the service provider is using to pay plan expenses came to the service provider either "directly or indirectly" from/through the placement of plan assets, then I think those funds are/should be plan assets as well. But I digress - because the DOL said the opposite. To that extent, if the ERISA bucket is NOT a plan asset, then I suppose, theoretically, they can be used by the plan sponsor for a different plan's expenses.
The service provider I work for does have ERISA accounts that are NOT plan assets - but we do, by contract, restrict their use only to paying expenses of the plan that generated those funds - but....