First and foremost, it's not an IRS issue. It's a DOL issue where those amounts are treated (by the DOL) as plan assets as soon as they are withheld from the participants' pay. AND, like any plan asset, the DOL is pretty adamant that they are separated from the employer's general assets (i.e. checking account) as soon as administratively feasible. With that in mind, two reasonable alternatives would be to have the recordkeeping platform to create a single account (e.g. forfeiture or whatever) for the exclusive purpose of receiving the contribution from the employer and holding it until it can get allocated pursuant to each participants' respective investment elections. The platform "MAY" have a lockbox account. Depending on the amount of time, the employer may write a check for the payroll amount and mail it into the platform's lock box. By the time it gets cashed and allocated, the accounts may be set up. Another approach may be for the employer to set up a checking account in the name of the plan for purposes of receiving the deposit. This may be least advisable if we're talking about a delay of a week or less. The thinking is merely to get those amounts separated from the employers assets as soon as administratively feasible. Whatever you do, ensure each participant's account receives the amount that was withheld from their pay.
Good Luck!