Recommendation for lightweight algorithm for partial message encryption
It's trivial to code your own wrapper function if you really want to use such API. Here is a Python-inspired pseudo code:

def mywrapper(data, offset, encspecs):
    subdata = data[0:offset]
    ciphertext = whatever_enc_function(subdata, encspecs)
    return ciphertext

encspecs = ... 
plaintext = "...etc"
ciphertxt = mywrapper(plaintext, offset, encspecs)
cipher_plain_mixed = ciphertxt + plaintext[offset:]


But either way, it's not an algorithmic/methodology problem as much as it is an implementation.

Side note: I sense that you are about to allow for replay attacks, or other similar ones. Make sure to include a unique initialization vector (e.g. a simple counter or timestamp) that is always different, so that you never (extremely rarely) get same cipher again even when the content plaintext is repeated. This prevents an attacker from fraudulently re-sending a previously captured block of encrypted data.