Oooh ok. This springs to mind:
You could allocate your whole overdraft amount to a category called 'expenses buffer' each month.
Then, you have a category called 'expenses' which you allocate money to from the 'expenses buffer'.
So, an example:
You have in your account £3000. But your available spending is, say, £8000 because your OD is £5000.
You enter into your bank account a deposit which matches your OD limit. (Bear with me).
So your 'balance' on that account is 'amount in account + amount available as OD'.
On day 1 your budget would look like this:
Available to spend: £8000
Then, you would allocate first and foremost, the amount that is your overdraft, into your 'expenses buffer':
Expenses buffer: £5000
Expenses: £0
Available to Budget: £3000
Which is right, because your real money is only £3000.
When you have an expense, say £200 hotel bill, you do this:
Expenses buffer: -£200= £4800
Expenses: +£200= £200
The hotel bill is categorised as 'expenses'.
This leaves your real money at £3000, because you're using your OD for those until they're reimbursed.
If you then have to spend £500 on flights, you do this:
Expenses buffer: -£500=£4300
Expenses: +£500=£700
ATB will still be £3000.
This method will only work if you don't spill over your OD into Your money for expenses. Well, it would still work, but you'd start to get a negative number in your buffer and then your ATB would go down.
I use this method for a savings account which I like to have 'on budget' but I don't like the money in it affecting the rest of my budget. So I have my DD's DLA paid in to it, and I budget the whole amount to a category called 'DD's DLA'. Then, if I need to spend any of it, I reduce the amount budgeted to 'DD's DLA' by the amount I need to spend.