Here is an inscription from Livno, Bosnia.  Now this is 2 or 3 centuries later but I want to point out the continuing interplay of "pagan" letter forms and the otherwise standard Old Cyrillic.  The text seen here reads "СЕ ЛЕЖИТ П[О]ПЪ ТѢХОДР[АГ]... РѢДЪЕДИНѢМЪ ГОД...".

For the "yats" (Ѣ), the writer has used a symbol resembling the alchemy sign for phosphorous   (composed of the element of fire triangle and the equilateral cross) which is identical to an Old Bulgarian sign from the Pliska Rosette. The same symbol is used for both mars and mercury.  The downward hooking arms of the "yats" are found in other old pagan letters - including the ancient Vinca Danube region script.  

That undeciphered writing system also shares the "two hockey sticks" shape which serves as a "X" in "ТѢХОДРАГ".  In Lithuania, these are seen as two sheathes of wheat, the symbol of Jumis, a fertility and harvest deity.  Because of its similarity to the Greek Chi derived "X" of cyrillic, the old pagan-era symbol is used.  

The Ж (which in its traditional shape is the same as the symbol for sal ammoniac and  the planet uranus)  is instead shaped like two tridents put end to end, something like a pisces  sign turned 90 degrees.

The Life of Constantine tells us that Constantine "mastered the whole of grammar" and then studied "Homer and geometry, and with Leo and Photius did dialectics and all the philosophy, besides rhetorics, and arithmetics and astronomy, and music, and all other Hellenic arts."  Its important to note that Constantine studied astronomy.  If he did use these "alchemy" symbols, it was not with their chemical meanings but probably because he was familiar with their planetary identities.  The Pliska Rosette is thought to represent the seven planets known to the early Bulgars.  This is where the overlap of Constantine's classical education and pagan wisdom can be seen.

His studies of Homer, also, may have led him to the belief that the ancient pagan writings of the barbarians could be a remnant of the highly developed ancient civilization of which Troy was the center.  In Byzantine times, "Scythian" usually referred to barbarians from north of the black sea who fought on horseback.  This includes the Proto-Bulgars, the Huns, the Alans, and the Avars - not all Indo Iranian peoples, but seen by the Greeks as having adopted much of the Scythian way of life and religion.  Troy was renowned as a center of horse breeding before the Trojan war.  The horse had great importance in the cult of Troy, as it did in "scythian" cultures.

In addition to using astronomical signs that represent planets or constellations, Constantine used several shapes resembling other astronomical concepts.  The glagolitic letters V, G, and Dlook a hell of a lot like astrological symbols for the movement of heavenly bodies.  V= setting, D=rising, G=something a little more complicated to explain here.  And these are only theories.  

But we know Cyril studied astronomy, which shares the ball and curved line constructions with Glagolitsa.