Well there are many types of capital. Fixed capital attached to land doesn't readily flow across borders unless owners spend money to disassemble and salvage it. Financial capital isn't really capital as banks can create money on security of fictitious capital without finding existing deposits for borrowers to purchase speculative assets which do not convey ownership of any fixed capital.

Allowing cross-border flows of floating capital such as machines and commodity inputs and outputs should not be problematic if all new legal tender is initially placed into circulation through public local loans issued on security of the replacement cost of industrial fixed capital physically attached to domestic soil.

Conveyance of state enforced privilege such as deeds, titles, patents can also only be recorded between domestic residents and domestically chartered corporations liable for corporate tax. While these privileges provide security for capital investment they are not really capital themselves.

Allowing foreign investors to purchase shares of domestic corporations does not seem overly problematic as long any deeds, titles, patents held by corporations are directly taxed and there are some distributive offsets and deductions for domestic residents.