They do exist but are very expensive. Basically by the time you go to them they know you are screwed and in desperate IRS-caused need of liquidity. So while the actual estate tax may be 40%, you wind up paying 50% or more. This is usually a debt deal, not an equity one - who wants shares in a closely held family corporation?

For developers at unicorns I think services are somewhat better (e.g. secondmarket, elite crowdfunder).