I think that under your situation, initially the “Sales Invoices” should be posted to a BS Liability account as in reality you are receiving a series of deposits for a future event and if its cancelled then you will be refunding deposits, not incurring negative sales.
So I suggest that:
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under
you activate 
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go to Settings - Chart of accounts and on the Balance Sheet create a new account called “Future Events”, select group as Liability and tick the Control Account made up of Special Accounts
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go to the Special Accounts and create separate accounts for each future event
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the Sales Invoice now gets posted to that account
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so now the Accounts Receivable for Twiddle will show a balance which will reduce with each deposit received.
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once the event has occurred, you will then do a Journal to transfer the “sale” from the Special Account to the P&L Sales account so your income matches the timing of the event
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also, looking at the Future Events account balance will indicate how much work you have booked ahead