Did you add it to the frr.conf or to frr.conf.local ? You should be able to copy the respective interface stanza from frr.conf to frr.conf.local and add the cost parameter to it as a work-around. We will also look into adding this option to the GUI.
You should be able to add something like this to frr.conf.local - then re-apply the SDN configuration. frr.conf.local gets merged with frr.conf and is left untouched by PVE. Make sure you copy the full generated interface section from the frr config.
Code:
interface <your_interface>
ip ospf area <your_area>
ip ospf network point-to-point
ip ospf cost 20
exit