Yes, activated Charcoal (AC) can be given rectally. Although this is a veterinary journal, I doubt the principles are different and according to this article it's given in a liquid slurry, much like oral administration:
Enemas also have been used to decrease the colonic bacterial numbers
and substrates. The following types of enemas have been recommended:
• Warm water enemas at 10 ml/kg q4-6h until signs improve, lactulose
enemas at 1 to 3 ml/10 kg BW diluted 1 : 1 to 1 : 3 with warm water
q6-8h
• Neomycin enemas at 15 to 20 ml of 1% solution q8-12h
• Metronidazole enemas at 7.5 mg/kg (systemic dose) mixed with water
q12h
• Betadine enemas given by diluting 1 : 10 with warm water and giving
10 ml/kg q8h and flushing out with warm water after 10 to 15 minutes
• Activated charcoal enemas using the liquid suspension q8h
• Vinegar enemas made by diluting the vinegar 1 : 4 with warm water
and administering at 10 ml/kg q8h
This is substantiated by the following human clinical trial involving the treatment of anal fistulas. Although I can't find historical evidence that rectal administration was a common practice in the past, it received new attention in 2017 with this study:
An open prospective study evaluating efficacy and safety of a new medical device for rectal application of activated carbon in the treatment of chronic, uncomplicated perianal fistulas.
The aim of this study was to evaluate the clinical effects on
non-Crohn’s disease perianal fistula healing, and the safety and
tolerability of a new medical device that applies high-purity,
high-activity granular activated carbon locally into the rectum twice
daily of patients with perianal fistulas without any concomitant
medication.
It appeared to be moderately successful:
Of 28 patients included, 10 patients (35.7%) showed complete fistula
healing (closed, no discharge on palpation) after 8 weeks; seven of
these patients, corresponding to 25% of the enrolled patients,
remained in remission for up to 31 weeks.
As I said before, I couldn't find historical evidence of rectal administration being used, but I did find an article from 1964 that seems to hint that it could have been administered other than orally in some cases. Unfortunately, the full text of the article is behind a pay wall. Here is what's freely available: