Just wondered if there are any. I don’t think so.
Tablets, you can feed via various means, with GDL90 WIFI being probably the most obvious route today.
One could generate a GPS signal and connect via a coax, but it’s pretty complicated. There is this using an SDR.
Dynon Skyview series via ARINC429?
OK, but that is not IFR certified. VFR avionics can do what they like. The old KMD150, in the version without internal GPS, also had NMEA input, but a box without GPS is really just a display so must have an input of some sort…
I’ve updated the subject line 
Probably some of these products have position data inputs. But being non-TSO they got certified along with the airframes, and AFAIK never for IFR (and incidentally none can display Jepp IAP data).
ARINC743 / ARINC743A is the obvious way to do it. It is a documented set of ARINC429 packets e.g. labels 110 and 111 and even Garmin have not made ARINC429 proprietary (although in their ARINC743A they have an undocumented requirement for a specific packet ordering). Both the GTX330ES and the TAS605A accept ARINC743/A (for ADS-B OUT and IN respectively) and that all works 100%, but these are not “navigators”.
Probably the lack of any input (other than GPS antenna) is a certification requirement. Also an SBAS box could not tell if SBAS is being received, other than via a direct GPS input, so e.g. LPV (or the different modes inherent with SBAS) could not be provided. Well, they could be, but that’s another discussion 
Various Garmin GDUs use an external GPS position source.
There are such systems in turboprops, bizjets and above. The Honeywell NZ2000 FMS accepts inertial data, DME and GPS from external sensors, as does the Collins Proline 21 or Fusion.
Dynon Skyview is as much IFR certified as Garmin G3x,
As Matt said, It will accept position data vie RS232 in NMEA,
My neighbour is using the Microair dual band GPS as a reliable sat nav
Neither is IFR. The G3X states
Add LPV/LNAV/ILS approach capabilities when you pair G3X Touch with select Garmin navigators (each sold separately), such as GTN 650Xi, GTN 750Xi, GNX 375, GNC 355 or GPS 175.
Absence of instrument approach capability does not mean “no IFR”
Peter wrote:
Neither is IFR. The G3X states
The instrumentation and display in a G3X system is IFR approved. Only the GPS is not.
