invoicing non-gainful *expenses* for reimbursement

[cognitive.libertarian+ml at gmail.com]

I have some material expenses to pass on to a customer, without
applying a markup.  I would like to have an invoiced item look like
this, for example:

  lumber   materal   expenses:lumber   50   $10    $500

The problem is, GC is *forcing* the transaction to use an *income*
account.  This makes no sense when you don't gain from simply passing
along an expense.  I want to be able to browse the expenses:lumber
account, and see -50 for my cost, followed by +50 for the
reimbursement (this ensures that the balance for net lumber costs is
accurate).

The actual scenario involves a reimbursement of expenses from many
different expense accounts.  These are reimbursements that rarely
happen, and in fact may not recur again.  It would be quite messy to
create an income account to mirror every expense account; which would
result in accounts with just one or two transactions, and expense
account balances that do not show the true net figure.

The hack that's tempting at the moment is to simply configure
"expense" accounts as "income" accounts, while still keeping them
children of a parent named "Expense".  This would enable the
invoicing to work with these accounts.

What's the general approach in this scenario?