What are some resources on Mahayana forms of Meditation? Are there any sources about Meditations of Mahayana Schools?
For example, a great classical source on zazen would be Bendōwa by Dogen, which people jokingly pronounce bend over.

If you include Vajrayana under Mahayana umbrella, then there are bits on tantric meditation in Introduction to Tantra on Lama Thubten Yeshe, or Guru Yoga by Tsongkhapa, which is a manual. In the same subject of Samatha there is great treatise commentary in Alan Wallace's Stilling the Mind: Shamatha Teachings from Dudjom Lingpa's Vajra Essence from perspective of Dzogchen. There will be companion book by Alan Wallace Fathoming the Mind: Inquiry and Insight in Dudjom Lingpa's Vajra Essence that takes on another section (Vipassana) of Vajra Essence treaty.

Apart from that, many references directly pointing at texts can be found in this section of Buddhist meditation on Wikipedia.