There is a possibility that *bast- itself is a loan into Germanic from Iranian, in which case there would be no difference between a Germanic and an Iranian explanation.
****GK: This is worth pursuing. Let's put our heads together on it.
Point n.1 The earliest known mention of "Bastarnae" is found in Polybius (H. 25.6.2-4) . This was written in the second half of the 2nd c. BCE, and refers to events of ca. 180-179 BCE
Point n. 2 The Protogenes Decree (ca. 200 BCE at the latest, but probably a little earlier) doesn't know of "Bastarnians" but only of an alliance of "Sciri" and "Galatae".
First question: Who would Philip and Perseus (Macedonians) have gotten the expression from? Hypothesis: from the Scythians of Scythia Minor. A contingent left behind by the major Scythians (in 180 BCE they would still be in Central Asia: cf. analogy of the Middle East epos in Herodotus!), ruled by a "king" (we have numismatic evidence), and "frightened" (uneasy and fearful) of incoming "Sciri" and "Galatae" acc. to the Protogenes decree. Another reason for the term: the S+G not only positioned themselves as a power in the sacred Scythian territory the contingent could not wholly defend (again ther Herodotus analogy), but theu came from an area (Poland) which the classical Scythians had frequently raided for slaves in their glory days (cf. the last phase of the Lusatian culture). So.... "Bastarnians" acc. to your view.
Second question: This term had a neutral (allies) and a "nasty" (slaves) connotation. Perhaps the former was presented to Philip by his interpreters in 180 BCE?