I was introduced to the Casket Seal Dharani last year, when a number of śarīra were passed to me. I inquired for the proper way to make offerings to these śarīra, and was suggested that in addition to offering the normal water, fruits flowers and incense I could also chant this dharani.
Back then I asked the question that I'm sure some of you would have in your minds - in which sutra reference does it say the Casket Seal Dharani is good for making an offering to śarīra? The reply referred me to the dharani's title and properties - "Ārya sarva tathāgata adhiṣṭhāna hṛdaya guhya dhātu karaṇḍa mudrā nāma dhāraṇī". The word "dhātu" means whole-body śarīra. The whole title literally means "the dharani of the seal of the casket holding the śarīra of all Buddha's secret whole-bodies". This dharani contains the śarīra of all Buddha of past and present. By chanting the dharani I am already making an offering to the śarīra of all Buddha. With or without the physical śarīra on my altar, I can still become one with the śarīra of all Buddha. I can become one with all Buddha, past and present. Doesn't that trump merely offering water, fruits, flowers, license or even the most precious gemstones and earthly treasures?
Hence, don't worry so much about receiving the Buddha's physical śarīra. Reading the Casket Seal Dharani and the sutra can make you be one with the Buddha's śarīra. Even if you don't read this sutra, that's OK - all sutra spoken by the Buddha are the śarīra of his Dharma body. The best way to make offering to the Buddha is to follow his teaching.
Namas triya-dhvikānāṃ sarva tathāgatānām!