Several barriers exist and the solution depends on the barrier we face. For example mixed pollen (compatible and incompatible) is used to overcome stimgmatic inhibitions. It is highly situation specific. The first thing is to know what type of barrier we are dealing with. Several strategies are available, we can employ anyone suitable for our requirement.
The type of barrier varies, but Lilium review by van Tuyl et al. can be a generic pathway, https://www.researchgate.net/profile/Jaap_Tuyl/publication/222482901_Application_of_in_vitro_pollination_ovary_culture_ovule_culture_and_embryo_rescue_for_overcoming_incongruity_barriers_in_interspecific_Lilium_crosses/links/0f3175370713fc92cc000000.pdf
Article Application of in vitro pollination, ovary culture, ovule cu...