Usage of whileas instead of whereas

OED does have a definition for "whileas" under the entry for "while," often used as two separate words. You are correct that it means the same as "whereas" and is both obsolete and more rare than "whereas" in archaic form; A modern English speaker would likely be confused by hearing it in casual conversation.

4. while as (also occasionally as one word, cf. whenas adv. and conj., whereas adv. and conj.).

The most recent use cited by OED is from a poem from 1918, and the full text can be found in the link below:

How do the days press on, and lay

Their fallen locks at evening down,

Whileas the stars in darkness play

And moonbeams weave a crown

• Walter De la Mare · Motley and other poems · 1918.

The oldest attestation is from 1563.

1563 Whyle as the rauenyng Wolues he prayed his gylteles lyfe to saue.

• Barnabe Googe · Eglogs epytaphes, and sonettes · 1st edition, 1563 (1 vol.).