A lot of men see it as money for the mother, as her being greedy. There are loads of ways to reduce or hide payments - upping pension contributions, for example.
I think there's a lot that could be done. It would take work and cost money - but if you could define avoidance in the same way as tax avoidance or deprivation of assets (for benefits payments) and penalise it, that would help. If you can make it not worthwhile to avoid paying, it would probably stop. If you can make it shameful on top of that, so much the better.
If the DWP have the power to look at benefits clsimants' bank accounts, let them look at non-paying fathers' accounts (including joint accounts) and work out a 'deemed lifestyle expenditure' and use that as the basis.
I like the idea of it remaining as an accrued debt. For some women, it wouldn't be so much use. But many women are using the money they earn to cover all the costs of raising children rather than putting it into mortgage payments or pension contributions - if you could receive it later in life you could pay a chunk off the mortgage or put it in your pension. Take it from their pension pot when the last child turns 18.