Is it possible for plant breeders to develop a true-breeding pink-flowered snapdragon?

No.

Explanation:

Pink flower colour in snapdragon is as a result of incomplete dominance. The pink flower colour has no allele in nature. Instead, the pink colour results from incomplete dominance of the allele that codes for red flower colour over the allele that codes for the white flower colour.

Assuming that the allele for red flower colour in the plant is R and the allele for white flower colour is r, true breeding red and white flower colour plants will have the genotype RR and rr respectively. In heterozygote state (Rr), the phenotype is usually pink, showing that the R allele has incomplete dominance over the r allele.

Hence, there is no way for plant breeders to develop a true-breeding pink-flowered snapdragon. without some genetic manipulations/mutation.