Extend TaxLine with an additional optional Many2One('account.tax', ...) field that could be used for tax reporting, similar in some respects to how analytic accounts work with financial accounts. This additional tax would have the same amount as each component tax. Some US states require taxes to be reported using a composite reporting tax code (usually at the level of the most local tax jurisdiction) rather than reporting the individual components that were used to figure the composite tax.
This functionality could be packaged as part of the above module or a separate module if this functionality is more broadly useful.